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  

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  

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  

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 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  

"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".