"I said to Abu Sa'eed: 'One of us performs Salat and we do not know how (much) we prayed.' So he said: 'Messenger of Allah (S) said: "When one of you performs Salat and he does now know how (much) he prayed then let him perform two prostrations while he is sitting."  

قُلْتُ لأَبِي سَعِيدٍ أَحَدُنَا يُصَلِّي فَلاَ يَدْرِي كَيْفَ صَلَّى فَقَالَ قَالَ رَسُولُ اللَّهِ ﷺ إِذَا صَلَّى أَحَدُكُمْ فَلَمْ يَدْرِ كَيْفَ صَلَّى فَلْيَسْجُدْ سَجْدَتَيْنِ وَهُوَ جَالِسٌ قَالَ وَفِي الْبَابِ عَنْ عُثْمَانَ وَابْنِ مَسْعُودٍ وَعَائِشَةَ وَأَبِي هُرَيْرَةَ  

Messenger of Allah (S) said: "Indeed the Shaitan comes to one of you in his Salat confusing him until he does now know how much he has prayed. When one of you experiences that then let him perform two prostrations while sitting."  

قَالَ رَسُولُ اللَّهِ ﷺ إِنَّ الشَّيْطَانَ يَأْتِي أَحَدَكُمْ فِي صَلاَتِهِ فَيَلْبِسُ عَلَيْهِ حَتَّى لاَ يَدْرِي كَمْ صَلَّى فَإِذَا وَجَدَ ذَلِكَ أَحَدُكُمْ فَلْيَسْجُدْ سَجْدَتَيْنِ وَهُوَ جَالِسٌ  

"When one of you becomes forgetful in his Salat so that he does not know if he prayed one or two, then let him consider it one. When he does not know if he prayed three or four, then let him consider it three, and let him perform two prostrations before he says the Taslim."  

سَمِعْتُ النَّبِيَّ ﷺ يَقُولُ إِذَا سَهَا أَحَدُكُمْ فِي صَلاَتِهِ فَلَمْ يَدْرِ وَاحِدَةً صَلَّى أَوْ ثِنْتَيْنِ فَلْيَبْنِ عَلَى وَاحِدَةٍ فَإِنْ لَمْ يَدْرِ ثِنْتَيْنِ صَلَّى أَوْ ثَلاَثًا فَلْيَبْنِ عَلَى ثِنْتَيْنِ فَإِنْ لَمْ يَدْرِ ثَلاَثًا صَلَّى أَوْ أَرْبَعًا فَلْيَبْنِ عَلَى ثَلاَثٍ وَلْيَسْجُدْ سَجْدَتَيْنِ قَبْلَ أَنْ يُسَلِّمَ