The Messenger of Allah ﷺ said: When any one of you is in doubt about his prayer and he does Dot know how much he has prayed, three or four (rak'ahs). he should cast aside his doubt and base his prayer on what he is sure of. then perform two prostrations before giving salutations. If he has prayed five rak'ahs, they will make his prayer an even number for him, and if he has prayed exactly four, they will be humiliation for the devil.  

قَالَ رَسُولُ اللَّهِ ﷺ إِذَا شَكَّ أَحَدُكُمْ فِي صَلاَتِهِ فَلَمْ يَدْرِ كَمْ صَلَّى ثَلاَثًا أَمْ أَرْبَعًا فَلْيَطْرَحِ الشَّكَّ وَلْيَبْنِ عَلَى مَا اسْتَيْقَنَ ثُمَّ يَسْجُدُ سَجْدَتَيْنِ قَبْلَ أَنْ يُسَلِّمَ فَإِنْ كَانَ صَلَّى خَمْسًا شَفَعْنَ لَهُ صَلاَتَهُ وَإِنْ كَانَ صَلَّى إِتْمَامًا لأَرْبَعٍ كَانَتَا تَرْغِيمًا لِلشَّيْطَانِ  

The Prophet ﷺ said: "If one of you does not know whether he prayed three or four (rak'ahs), let him pray a rak'ah then prostrate twice after that when he is sitting. Then if he prayed five (rak'ahs), they (the two prostrations) will make his prayer even-numbered, and if he had prayed four, they will annoy and humiliate the shaitan."  

عَنِ النَّبِيِّ ﷺ قَالَ إِذَا لَمْ يَدْرِ أَحَدُكُمْ صَلَّى ثَلاَثًا أَمْ أَرْبَعًا فَلْيُصَلِّ رَكْعَةً ثُمَّ يَسْجُدْ بَعْدَ ذَلِكَ سَجْدَتَيْنِ وَهُوَ جَالِسٌ فَإِنْ كَانَ صَلَّى خَمْسًا شَفَعَتَا لَهُ صَلاَتَهُ وَإِنْ صَلَّى أَرْبَعًا كَانَتَا تَرْغِيمًا لِلشَّيْطَانِ  

when one of you is in doubt about his prayer (i.e, how much he has prayed), he should throw away his doubt and base his prayer on what he is sure of. When he is sure about the completion of his prayer, he should make two prostrations (at the end of the prayer). If the prayer is complete, the additional rak’ah and the two prostrations will be supererogatory prayer. If the prayer is incomplete, the additional rak’ahs will compensate it, and the two prostrations will be a disgrace for the devil.  

قَالَ رَسُولُ اللَّهِ ﷺ إِذَا شَكَّ أَحَدُكُمْ فِي صَلاَتِهِ فَلْيُلْقِ الشَّكَّ وَلْيَبْنِ عَلَى الْيَقِينِ فَإِذَا اسْتَيْقَنَ التَّمَامَ سَجَدَ سَجْدَتَيْنِ فَإِنْ كَانَتْ صَلاَتُهُ تَامَّةً كَانَتِ الرَّكْعَةُ نَافِلَةً وَالسَّجْدَتَانِ وَإِنْ كَانَتْ نَاقِصَةً كَانَتِ الرَّكْعَةُ تَمَامًا لِصَلاَتِهِ وَكَانَتِ السَّجْدَتَانِ مُرْغِمَتَىِ الشَّيْطَانِ  

“The Messenger of Allah ﷺ said: ‘If anyone of you is uncertain about his prayer, let him put aside uncertainty and act upon that which is certain. When he has made sure his prayer is complete, then let him prostrate twice. Then if his prayer was complete, that (extra) Rak’ah will be counted as voluntary, and if his prayer was lacking, that Rak’ah will complete his prayer, and the two prostrations will rub the Satan’s nose in the dust.’”  

قَالَ رَسُولُ اللَّهِ ﷺ إِذَا شَكَّ أَحَدُكُمْ فِي صَلاَتِهِ فَلْيُلْغِ الشَّكَّ وَلْيَبْنِ عَلَى الْيَقِينِ فَإِذَا اسْتَيْقَنَ التَّمَامَ سَجَدَ سَجْدَتَيْنِ فَإِنْ كَانَتْ صَلاَتُهُ تَامَّةً كَانَتِ الرَّكْعَةُ نَافِلَةً وَإِنْ كَانَتْ نَاقِصَةً كَانَتِ الرَّكْعَةُ لِتَمَامِ صَلاَتِهِ وَكَانَتِ السَّجْدَتَانِ رَغْمَ أَنْفِ الشَّيْطَانِ  

Yahya related to me from Malik from Zayd ibn Aslam from Ata ibn Yasar that the Messenger of Allah ﷺ said, "If you are uncertain in the prayer and do not know whether you have prayed three or four rakas, then pray a raka and make two prostrations from the sitting position before the taslim. If the raka that you prayed was the fifth, then you make it even by these two sajdas, and if it was the fourth, then the two prostrations spite Shaytan."  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ إِذَا شَكَّ أَحَدُكُمْ فِي صَلاَتِهِ فَلَمْ يَدْرِ كَمْ صَلَّى أَثَلاَثًا أَمْ أَرْبَعًا فَلْيُصَلِّي رَكْعَةً وَلْيَسْجُدْ سَجْدَتَيْنِ وَهُوَ جَالِسٌ قَبْلَ التَّسْلِيمِ فَإِنْ كَانَتِ الرَّكْعَةُ الَّتِي صَلَّى خَامِسَةً شَفَعَهَا بِهَاتَيْنِ السَّجْدَتَيْنِ وَإِنْ كَانَتْ رَابِعَةً فَالسَّجْدَتَانِ تَرْغِيمٌ لِلشَّيْطَانِ  

[Machine] The Messenger of Allah ﷺ said: "If anyone of you has doubt in their prayer and does not know whether they prayed three or four [rak'ahs], they should disregard their doubt and proceed according to what they are certain about. Then they should perform two prostrations before concluding their Salah. Because if they prayed five [rak'ahs], it will make up for their prayer as a whole." Musa said once, "If they prayed five [rak'ahs], they would make up for their prayer. And if they completed four [rak'ahs], they would prevent Satan from harm."  

قَالَ رَسُولُ اللهِ ﷺ إِذَا شَكَّ أَحَدُكُمْ فِي صَلَاتِهِ فَلَمْ يَدْرِ كَمْ صَلَّى ثَلَاثًا أَمْ أَرْبَعًا فَلْيَطْرَحِ الشَّكَّ وَلْيَبْنِ عَلَى مَا اسْتَيْقَنَ ثُمَّ يَسْجُدْ سَجْدَتَيْنِ قَبْلَ أَنْ يُسَلِّمَفَإِنْ كَانَ صَلَّى خَمْسًا كَانَتْ شَفْعًا لِصَلَاتِهِ قَالَ مُوسَى مَرَّةً فَإِنْ كَانَ صَلَّى خَمْسًا شَفَعْنَ لَهُ صَلَاتَهُ وَإِنْ كَانَ صَلَّى إِتْمَامَ أَرْبَعٍ كَانَتَا تَرْغِيمًا لِلشَّيْطَانِ  

[Machine] On the authority of the Prophet ﷺ, he said: If one of you has doubt during prayer and does not know whether they have prayed three or four (units), then they should get up and perform one unit of prayer (rakah). He said: Increase (the number of units of prayer) until the doubt is in the increase. Then, they should prostrate for the two prostrations of forgetfulness. If they prayed five (units of prayer), it will make up for their prayer. And if they prayed four, they will be compelled by the Satan.  

عَنِ النَّبِيِّ ﷺ قَالَ إِذَا شَكَّ أَحَدُكُمْ فِي الصَّلَاةِ فَلَمْ يَدْرِ ثَلَاثًا صَلَّى أَمْ أَرْبَعًا فَلْيَقُمْ فَلْيُصَلِّ رَكْعَةً قَالَ يَزِيدُ حَتَّى يَكُونَ الشَّكُّ فِي الزِّيَادَةِ ثُمَّ لِيَسْجُدْ سَجْدَتَيِ السَّهْوِ فَإِنْ كَانَ صَلَّى خَمْسًا شَفَعَتَا لَهُ صَلَاتَهُ وَإِنْ كَانَ صَلَّى أَرْبَعًا فَهُمَا يُرْغِمَانِ الشَّيْطَانَ  

[Machine] The Messenger of Allah ﷺ said: "If one of you has doubts about his prayer and does not know how many units he has prayed, then he should rely on certainty until he is certain that he has completed it. Then, he should prostrate for two extra prostrations before saying the tasleem (greeting). For if his prayer was odd (witr), it will become even (shaf') by doing so, and if it was already even, then that would be a rebuke to the devil."  

أَنَّ رَسُولَ اللهِ ﷺ قَالَ إِذَا شَكَّ أَحَدُكُمْ فِي صَلَاتِهِ فَلَمْ يَدْرِ كَمْ صَلَّى فَلْيَبْنِ عَلَى الْيَقِينِ حَتَّى إِذَا اسْتَيْقَنَ أَنْ قَدْ أَتَمَّ فَلْيَسْجُدْ سَجْدَتَيْنِ قَبْلَ أَنْ يُسَلِّمَ فَإِنَّهُ إِنْكَانَتْ صَلَاتُهُ وِتْرًا صَارَتْ شَفْعًا وَإِنْ كَانَتْ شَفْعًا كَانَ ذَلِكَ تَرْغِيمًا لِلشَّيْطَانِ  

“The Messenger of Allah ﷺ said: ‘If anyone of you is uncertain about his prayer, let him put aside uncertainty and act upon that which is certain. When he has made sure his prayer is complete, then let him prostrate twice. Then if his prayer was complete, that (extra) Rak’ah will be counted as voluntary, and if his prayer was lacking, that Rak’ah will complete his prayer, and the two prostrations will rub the Satan’s nose in the dust.’” (Using translation from Ibn Mājah 1210)   

قَالَ رَسُولُ اللَّهِ ﷺ «إِذَا شَكَّ أَحَدُكُمْ فِي صَلَاتِهِ فَلْيُلْقِ الشَّكَّ وَلْيَبْنِ عَلَى الْيَقِينِ فَإِنِ اسْتَيْقَنَ التَّمَامَ سَجَدَ سَجْدَتَيْنِ فَإِنْ كَانَتْ صَلَاتُهُ تَامَّةً كَانَتِ الرَّكْعَةُ نَافِلَةً وَالسَّجْدَتَانِ وَإِنْ كَانَتْ نَاقِصَةً كَانَتِ الرَّكْعَةُ تَمَامًا لِصَلَاتِهِ وَالسَّجْدَتَانِ يُرْغِمَانِ أَنْفَ الشَّيْطَانِ»  

The Prophet ﷺ said: When one of you is in doubt about his prayer, and does not know how much he has prayed, three or four rak'ahs, he should pray one (additional) rak'ah and make two prostrations while sitting before giving the salutation. If the (additional) rak'ah which he prayed is the fifth one, he will make it an even number by these two prostrations. If it is the fourth one, the two prostrations will be a disgrace for the devil. (Using translation from Abū Dāʾūd 1026)   

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ «إِذَا صَلَّى أَحَدُكُمْ فَلَمْ يَدْرِ ثَلَاثًا صَلَّى أَمْ أَرْبَعًا فَلْيُصَلِّ رَكْعَةً وَلْيَسْجُدْ سَجْدَتَيْنِ قَبْلَ السَّلَامِ فَإِنْ كَانَتْ ثَالِثَةً شَفَعَتْهَا السَّجْدَتَانِ وَإِنْ كَانَتْ رَابِعَةً فَالسَّجْدَتَانِ تَرْغِيمٌ لِلشَّيْطَانِ»  

“The Messenger of Allah ﷺ said: ‘If anyone of you is uncertain about his prayer, let him put aside uncertainty and act upon that which is certain. When he has made sure his prayer is complete, then let him prostrate twice. Then if his prayer was complete, that (extra) Rak’ah will be counted as voluntary, and if his prayer was lacking, that Rak’ah will complete his prayer, and the two prostrations will rub the Satan’s nose in the dust.’” (Using translation from Ibn Mājah 1210)   

قَالَ رَسُولُ اللَّهِ ﷺ «إِذَا شَكَّ أَحَدُكُمْ فِي صَلَاتِهِ فَلْيُلْقِ الشَّكَّ وَلْيَبْنِ عَلَى الْيَقِينِ فَإِنِ اسْتَيْقَنَ التَّمَامَ سَجَدَ سَجْدَتَيْنِ فَإِنْ كَانَتْ صَلَاتُهُ تَامَّةً كَانَتِ الرَّكْعَةُ نَافِلَةً وَالسَّجْدَتَانِ نَافِلَةً وَإِنْ كَانَتْ نَاقِصَةً كَانَتِ الرَّكْعَةُ تَمَامًا لِصَلَاتِهِ وَالسَّجْدَتَانِ تُرْغِمَانِ أَنْفَ الشَّيْطَانِ»  

“The Messenger of Allah ﷺ said: ‘If anyone of you is uncertain about his prayer, let him put aside uncertainty and act upon that which is certain. When he has made sure his prayer is complete, then let him prostrate twice. Then if his prayer was complete, that (extra) Rak’ah will be counted as voluntary, and if his prayer was lacking, that Rak’ah will complete his prayer, and the two prostrations will rub the Satan’s nose in the dust.’” (Using translation from Ibn Mājah 1210)   

قَالَ رَسُولُ اللَّهِ ﷺ «إِذَا شَكَّ أَحَدُكُمْ فِي صَلَاتِهِ فَلْيُلْقِ الشَّكَّ وَلْيَبْنِ عَلَى الْيَقِينِ فَإِنِ اسْتَيْقَنَ التَّمَامَ سَجَدَ سَجْدَتَيْنِ فَإِنْ كَانَتْ صَلَاتُهُ تَامَّةً كَانَتِ الرَّكْعَةُ نَافِلَةً وَالسَّجْدَتَانِ نَافِلَةً وَإِنْ كَانَتْ نَاقِصَةً كَانَتِ الرَّكْعَةُ تَمَامًا بِصَلَاتِهِ وَالسَّجْدَتَانِ تُرْغِمَانِ أَنْفَ الشَّيْطَانِ»  

The Prophet ﷺ said: When one of you is in doubt about his prayer, and does not know how much he has prayed, three or four rak'ahs, he should pray one (additional) rak'ah and make two prostrations while sitting before giving the salutation. If the (additional) rak'ah which he prayed is the fifth one, he will make it an even number by these two prostrations. If it is the fourth one, the two prostrations will be a disgrace for the devil. (Using translation from Abū Dāʾūd 1026)   

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ «إِذَا صَلَّى أَحَدُكُمْ فَلَمْ يَدْرِ ثَلَاثًا صَلَّى أَمْ أَرْبَعًا فَلْيُصَلِّ رَكْعَةً وَلْيَسْجُدْ سَجْدَتَيْنِ قَبْلَ السَّلَامِ فَإِنْ كَانَتْ رَابِعَةً فَالسَّجْدَتَانِ تَرْغِيمًا لِلشَّيْطَانِ وَإِنْ كَانَتْ خَامِسَةً شَفَعَتْهَا السَّجْدَتَانِ»  

[Machine] The Messenger of Allah ﷺ said, "If any one of you is in doubt about how many rak'ahs they have prayed, whether three or four, then they should stand up and perform a complete rak'ah, with its proper bowing and prostration, and then they should perform two prostrations while sitting. If they have already performed five rak'ahs, they should complete the prayer with these two prostrations. But if they have already performed four rak'ahs, these two prostrations are a form of humiliation for the devil."  

قَالَ رَسُولُ اللَّهِ ﷺ «إِذَا شَكَّ أَحَدُكُمْ فَلَمْ يَدْرِ كَمْ صَلَّى ثَلَاثًا أَوْ أَرْبَعًا فَلْيَقُمْ فَلْيُصَلِّ رَكْعَةً يُتِمُّ رُكُوعَهَا وَسُجُودَهَا ثُمَّ يَسْجُدْ سَجْدَتَيْنِ وَهُوَ جَالِسٌ فَإِنْ كَانَ قَدْ صَلَّى خَمْسًا شَفَعَ بِالسَّجْدَتَيْنِ وَإِنْ كَانَ قَدْ صَلَّى أَرْبَعًا كَانَتِ السَّجْدَتَانِ تَرْغِيمًا لِلشَّيْطَانِ»  

The Prophet ﷺ said: "If one of you does not know whether he prayed three or four (rak'ahs), let him pray a rak'ah then prostrate twice after that when he is sitting. Then if he prayed five (rak'ahs), they (the two prostrations) will make his prayer even-numbered, and if he had prayed four, they will annoy and humiliate the shaitan." (Using translation from Nasāʾī 1239)  

عَنِ النَّبِيِّ ﷺ قَالَ «إِذَا لَمْ يَدْرِ أَحَدُكُمْ كَمْ صَلَّى ثَلَاثًا أَمْ أَرْبَعًا فَلْيُصَلِّ رَكْعَةً ثُمَّ يَسْجُدْ بَعْدَ ذَلِكَ سَجْدَتَيْنِ وَهُوَ جَالِسٌ فَإِنْ كَانَ صَلَّى خَمْسًا شَفَعَتَا لَهُ صَلَاتَهُ وَإِنْ صَلَّى أَرْبَعًا كَانَتَا تَرْغِيمًا لِلشَّيْطَانِ»  

The Prophet ﷺ said: When one of you is in doubt about his prayer, and does not know how much he has prayed, three or four rak'ahs, he should pray one (additional) rak'ah and make two prostrations while sitting before giving the salutation. If the (additional) rak'ah which he prayed is the fifth one, he will make it an even number by these two prostrations. If it is the fourth one, the two prostrations will be a disgrace for the devil. (Using translation from Abū Dāʾūd 1026)   

أَنَّ رَسُولَ اللهِ ﷺ قَالَ «إِذَا شَكَّ أَحَدُكُمْ فِي صَلَاتِهِ فَلَمْ يَدْرِ أَصَلَّى ثَلَاثًا أَوْ أَرْبَعًا فَلْيَقُمْ فَلْيَرْكَعْ» يَعْنِي رَكْعَةً وَيَسْجُدُ سَجْدَتَيْنِ فَإِنْ كَانَتْ خَامِسَةً شَفَعَهَا بِسَجْدَتَيْنِ وَإِنْ كَانَتْ رَابِعَةٌ كَانَتِ السَّجْدَتَانِ تَرْغِيمًا لِلشَّيْطَانِ  

[Machine] The Messenger of Allah ﷺ said: "If one of you has doubt about whether he has prayed three or four units (rak'ahs), then let him pray a complete unit (rak'ah) and then make two prostrations while sitting. If that unit of prayer was the fifth, then he should make those two prostrations as a supplication. And if it was the fourth, then they serve as a deterrent to the devil." 'Uyad bin Hilal differed in his wording of the hadith.  

أَنَّ رَسُولَ اللهِ ﷺ قَالَ «إِذَا شَكَّ أَحَدُكُمْ فَلَمْ يَدْرِ أَصَلَّى ثَلَاثًا أَمْ أَرْبَعًا فَلْيُصَلِّ رَكْعَةً تَامَّةً ثُمَّ يَسْجُدْ سَجْدَتَيْنِ وَهُوَ جَالِسٌ فَإِنْ كَانَتِ تِلْكَ الرَّكْعَةُ خَامِسَةً شَفَعَ بِهَاتَيْنِ السَّجْدَتَيْنِ وَإِنْ كَانَتْ رَابِعَةً كَانَتَا تَرْغِيمًا لِلشَّيْطَانِ» خَالَفَهُ عِيَاضُ بْنُ هِلَالٍ فِي لَفْظِهِ  

[Machine] The Messenger of Allah ﷺ said: "If any of you has doubts about his prayer and does not know how many units he has prayed, then he should pray one unit (rak'ah) and prostrate twice while sitting before the salutation (salam). If the prayer he has prayed is his fifth unit (rak'ah), then he should perform these two prostrations with intention of seeking forgiveness (tawbah) from Satan. This narration has been narrated from the hadith of Sulayman bin Bilal and Hisham bin Saad from Zaid bin Aslam from Ata bin Yasar from Abu Sa'id, connected in the chain of narrators. This has also been narrated from the hadith of Malik, attributed to the same chain of narrators."  

أَنَّ رَسُولَ اللهِ ﷺ قَالَ إِذَا شَكَّ أَحَدُكُمْ فِي صَلَاتِهِ فَلَمْ يَدْرِ كَمْ صَلَّى ثَلَاثًا أَوْ أَرْبَعًا فَلْيُصَلِّ رَكْعَةً وَلْيَسْجُدْ سَجْدَتَيْنِ وَهُوَ جَالِسٌ قَبْلَ التَّسْلِيمِ فَإِنْ كَانَتِ الرَّكْعَةُ الَّتِي صَلَّى خَامِسَةً شَفَعَهَا بِهَاتَيْنِ وَإِنْ كَانَتْ رَابِعَةً فَالسَّجْدَتَانِ تَرْغِيمٌ لِلشَّيْطَانِ وَقَدْ رُوِّينَاهُ مِنْ حَدِيثِ سُلَيْمَانَ بْنِ بِلَالٍ وَهِشَامِ بْنِ سَعْدٍ عَنْ زَيْدِ بْنِ أَسْلَمَ عَنْ عَطَاءِ بْنِ يَسَارٍ عَنْ أَبِي سَعِيدٍ مَوْصُولًا وَقَدْ رُوِيَ مِنْ حَدِيثِ مَالِكٍ أَيْضًا مَوْصُولًا  

[Machine] The Messenger of Allah ﷺ said, "If any of you have doubt about his prayer, he should not know whether he prayed three or four units (rak'ahs). He should dismiss the doubt and be certain. Then he should perform two prostrations before making the tasleem (saying salaam at the end of the prayer). If it is an odd number of units, he should make it an intercessory one through these two prostrations, and if it is an even number, these two prostrations become a means of rebuke for Satan."  

قَالَ رَسُولُ اللهِ ﷺ إِذَا شَكَّ أَحَدُكُمْ فِي صَلَاتِهِ فَلَا يَدْرِي أَثْلَاثًا صَلَّى أَمْ أَرْبَعًا فَلْيُلْقِ الشَّكَّ وَلْيَبْنِ عَلَى الْيَقِينِ ثُمَّ لِيَسْجُدْ سَجْدَتَيْنِ قَبْلَ أَنْ يُسَلِّمَ فَإِنْ كَانَتْ وِتْرًا أَشْفَعَهَا بِهَاتَيْنِ السَّجْدَتَيْنِ وَإِنْ كَانَتْ شَفْعًا فَالسَّجْدَتَانِ تَرْغِيمٌ لِلشَّيْطَانِ  

when one of you is in doubt about his prayer (i.e, how much he has prayed), he should throw away his doubt and base his prayer on what he is sure of. When he is sure about the completion of his prayer, he should make two prostrations (at the end of the prayer). If the prayer is complete, the additional rak’ah and the two prostrations will be supererogatory prayer. If the prayer is incomplete, the additional rak’ahs will compensate it, and the two prostrations will be a disgrace for the devil. (Using translation from Abū Dāʾūd 1024)   

قَالَ رَسُولُ اللهِ ﷺ إِذَا شَكَّ أَحَدُكُمْ فِي صَلَاتِهِ فَلْيُلْقِ الشَّكَّ وَلْيَبْنِ عَلَى الْيَقِينِ فَإِذَا اسْتَيْقَنَ التَّمَامَ سَجَدَ سَجْدَتَيْنِ فَإِنْ كَانَتْ صَلَاتُهُ تَامَّةً كَانَتِ الرَّكْعَةُ نَافِلَةً وَالسَّجْدَتَانِ وَإِنْ كَانَتْ نَاقِصَةً كَانَتِ الرَّكْعَةُ تَمَامًا لِصَلَاتِهِ وَكَانَتِ السَّجْدَتَانِ مُرْغِمَتَيِ الشَّيْطَانِ  

"إِذا صلَّى أحدُكُمْ فلم يدرِ ثلاثًا صلى أم أربعًا؟ فْلُيصلِّ ركعةً ويسجُدْ سجدتين قَبل السلام، فإنْ كانَتْ ثالثةً شفعها السَّجدتان، وإنْ كانَتْ رابعةً فالسجدتان تَرْغيمٌ للشَّيطْان".  

The Messenger of Allah ﷺ said: When any one of you is in doubt about his prayer and he does Dot know how much he has prayed, three or four (rak'ahs). he should cast aside his doubt and base his prayer on what he is sure of. then perform two prostrations before giving salutations. If he has prayed five rak'ahs, they will make his prayer an even number for him, and if he has prayed exactly four, they will be humiliation for the devil. (Using translation from Muslim 571a)   

"إذا شكَّ أحدُكُمْ في صلاتِه فلمْ يَدْرِ كم صلَّى، ثلاثًا أمْ أربْعًا؟ فليَطرحْ الشَّكَ ولْيبْنِ على ما اسْتيقَنَ ثُمَّ يَسْجُدْ سجْدَتين قْبلَ أنْ يُسلِّمَ، فإِن كانَ صلَّى خمسًا شَفعْن له صَلاته، وإنْ كانَ صلَّى إتْمامًا لأربعٍ كانتا تَرْغيما للشَّيطان".  

when one of you is in doubt about his prayer (i.e, how much he has prayed), he should throw away his doubt and base his prayer on what he is sure of. When he is sure about the completion of his prayer, he should make two prostrations (at the end of the prayer). If the prayer is complete, the additional rak’ah and the two prostrations will be supererogatory prayer. If the prayer is incomplete, the additional rak’ahs will compensate it, and the two prostrations will be a disgrace for the devil. (Using translation from Abū Dāʾūd 1024)   

"إِذَا شكّ أحدُكُمْ في صَلاتِه فْليُلقِ الشَّكَ، وليَبْنِ على اليقينِ، فإِن استيقَنَ التمام سجدَ سجْدتين، فإِنْ كانتْ صلاتُه تامة كانتْ الركْعةُ نافلة والسجْدتان نافلةً، وإنْ كانت ناقصةً كانتْ الركعةُ تمامَ الصَلاةِ، والسجْدتان تُرْغمان أنْفَ الشَّيطان".