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  

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] 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  

[Machine] The Messenger of Allah ﷺ said, "If one of you is unsure whether he has prayed three or four (rak'ahs), then he should complete it and perform one more rak'ah, then make two prostrations of forgetfulness while sitting. If his prayer was five (rak'ahs), then the extra rak'ah is a voluntary prayer, and if it was four (rak'ahs), then they are a means of thwarting Satan, and this is their meaning."  

قَالَ رَسُولُ اللهِ ﷺ إِذَا لَمْ يَدْرِ أَحَدُكُمْ ثَلَاثًا صَلَّى أَمْ أَرْبَعًا؟ فَلْيُتِمَّ وَيُصَلِّي رَكْعَةً ثُمَّ يَسْجُدْ بَعْدَ ذَلِكَ سَجْدَتَيِ السَّهْوِ وَهُوَ جَالِسٌ فَإِنْ كَانَتْ صَلَاتُهُ خَمْسًا شَفَعَتْ صَلَاتَهُ وَإِنْ كَانَتْ أَرْبَعًا كَانَتَا تَرْغِيمًا لِلشَّيْطَانِ وَبِمَعْنَاهُ  

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".