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] On the authority of the Prophet, ﷺ , he said, "If anyone of you has doubt in his prayer, he should dispel the doubt and act upon certainty. Then, if he becomes certain, he should perform two prostrations while sitting."  

عَنِ النَّبِيِّ ﷺ قَالَ «إِذَا شَكَّ أَحَدُكُمْ فِي صَلَاتِهِ فَلْيُلْغِ الشَّكَّ وَلْيَبْنِ عَلَى الْيَقِينِ فَإِذَا اسْتَيْقَنَ بِالتَّمَامِ فَلْيَسْجُدْ سَجْدَتَيْنِ وَهُوَ قَاعِدٌ»  

[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  

“One of us prays and he does not know how many (Rak’ah) he has prayed.” He said: “The Messenger of Allah ﷺ said: ‘When anyone of you prays and does not know how many he has prayed, let him perform two prostrations while he is sitting.’” (Using translation from Ibn Mājah 1204)   

سَأَلْتُ أَبَا سَعِيدٍ الْخُدْرِيِّ قُلْتُ يُصَلِّي أَحَدُنَا فَلَا يَدْرِي كَمْ صَلَّى؟ قَالَ قَالَ رَسُولُ اللهِ ﷺ «إِذَا صَلَّى أَحَدُكُمْ فَلَمْ يَدْرِ كَمْ صَلَّى فَلْيَسْجُدْ سَجْدَتَيْنِ وَهُوَ جَالِسٌ»  

[Machine] The Messenger of Allah ﷺ said, "If one of you prays and forgets or does not know if they have added or missed something, they should perform two prostrations while sitting."  

قَالَ رَسُولُ اللهِ ﷺ «إِذَا صَلَّى أَحَدُكُمْ فَنَسِيَ» أَوْ قَالَ «فَلَمْ يَدْرِ زَادَ أَوْ نَقَصَ فَلْيَسْجُدْ سَجْدَتَيْنِ وَهُوَ جَالِسٌ»