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 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.  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ إِذَا شَكَّ أَحَدُكُمْ فِي صَلاَتِهِ فَلاَ يَدْرِي كَمْ صَلَّى ثَلاَثًا أَوْ أَرْبَعًا فَلْيُصَلِّ رَكْعَةً وَيَسْجُدْ سَجْدَتَيْنِ وَهُوَ جَالِسٌ قَبْلَ التَّسْلِيمِ فَإِنْ كَانَتِ الرَّكْعَةُ الَّتِي صَلَّى خَامِسَةً شَفَعَهَا بِهَاتَيْنِ 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.’”  

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  

“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 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»  

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  

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".