Sunan an-Nasa'i, Book 13, Hadith 64 (In-Book) ⓘ

It was narrated that 'Abdullah said: "The Messenger of Allah prayed and did more or less (rak'ahs). When he had said the taslim, it was said: 'O Messenger of Allah, has there been some change concerning the prayer?' He said: 'If there had been some change concerning the prayer, I would have told you. Rather I am a human being and I forget as you forget. If any one of you is not sure about his prayer, let him consider an estimate of what is correct, and complete his prayer on that basis, then say the taslim and prostrate twice.'"

أَخْبَرَنَا سُوَيْدُ بْنُ نَصْرٍ، قَالَ أَنْبَأَنَا عَبْدُ اللَّهِ، عَنْ مِسْعَرٍ، عَنْ مَنْصُورٍ، عَنْ إِبْرَاهِيمَ، عَنْ عَلْقَمَةَ، عَنْ عَبْدِ اللَّهِ، قَالَ صَلَّى رَسُولُ اللَّهِ صلى الله عليه وسلم فَزَادَ أَوْ نَقَصَ فَلَمَّا سَلَّمَ قُلْنَا يَا رَسُولَ اللَّهِ هَلْ حَدَثَ فِي الصَّلاَةِ شَىْءٌ قَالَ ‏ "‏ لَوْ حَدَثَ فِي الصَّلاَةِ شَىْءٌ أَنْبَأْتُكُمُوهُ وَلَكِنِّي إِنَّمَا أَنَا بَشَرٌ أَنْسَى كَمَا تَنْسَوْنَ فَأَيُّكُمْ مَا شَكَّ فِي صَلاَتِهِ فَلْيَنْظُرْ أَحْرَى ذَلِكَ إِلَى الصَّوَابِ فَلْيُتِمَّ عَلَيْهِ ثُمَّ لْيُسَلِّمْ وَلْيَسْجُدْ سَجْدَتَيْنِ ‏" ‏ ‏.‏