Sunan Ibn Majah, Volume 2, Book 5, Hadith 1203 (DarusSalam) ⓘ

It was narrated that ‘Abdullah said: “The Messenger of Allah prayed, and he added or omitted something.” (One of the narrators) Ibrahim said: “The confusion stems from me (i.e., he was not sure which it was).” “It was said to him: ‘O Messenger of Allah! Has something been added to the prayer?’ He said: ‘I am only human, I forget just as you forget. If anyone forgets, let him perform two prostrations when he is sitting (at the end).’ Then the Prophet turned and prostrated twice.”

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ عَامِرِ بْنِ زُرَارَةَ، حَدَّثَنَا عَلِيُّ بْنُ مُسْهِرٍ، عَنِ الأَعْمَشِ، عَنْ إِبْرَاهِيمَ، عَنْ عَلْقَمَةَ، عَنْ عَبْدِ اللَّهِ، قَالَ صَلَّى رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ فَزَادَ أَوْ نَقَصَ - قَالَ إِبْرَاهِيمُ وَالْوَهْمُ مِنِّي - فَقِيلَ لَهُ يَا رَسُولَ اللَّهِ أَزِيدَ فِي الصَّلاَةِ شَىْءٌ قَالَ ‏ "‏ إِنَّمَا أَنَا بَشَرٌ أَنْسَى كَمَا تَنْسَوْنَ فَإِذَا نَسِيَ أَحَدُكُمْ فَلْيَسْجُدْ سَجْدَتَيْنِ وَهُوَ جَالِسٌ ‏" ‏ ‏.‏ ثُمَّ تَحَوَّلَ النَّبِيُّ ـ صلى الله عليه وسلم ـ فَسَجَدَ سَجْدَتَيْنِ ‏.‏