Sahih Muslim , Book 5, Hadith 123 (In-Book) ⓘ

'Abdullah (b. Mas'ud) reported: The Messenger of Allah said prayer and he omitted or committed (something). Ibrahim (one of the narrators of this hadith) said: It is my doubt, and it was said: Messenger of Allah, has there been any addition to the prayer? He (the Holy Prophet) said: Verily I am a human being like you. I forget just as you forget so when any one of you forgets, he must perform two prostrations, and he (the Holy Prophet) was sitting and then the Messenger of Allah turned (his face towards the Qibla) and performed two prostrations.

وَحَدَّثَنَا مِنْجَابُ بْنُ الْحَارِثِ التَّمِيمِيُّ، أَخْبَرَنَا ابْنُ مُسْهِرٍ، عَنِ الأَعْمَشِ، عَنْ إِبْرَاهِيمَ، عَنْ عَلْقَمَةَ، عَنْ عَبْدِ اللَّهِ، قَالَ صَلَّى رَسُولُ اللَّهِ صلى الله عليه وسلم فَزَادَ أَوْ نَقَصَ - قَالَ إِبْرَاهِيمُ وَالْوَهْمُ مِنِّي - فَقِيلَ يَا رَسُولَ اللَّهِ أَزِيدَ فِي الصَّلاَةِ شَىْءٌ فَقَالَ ‏ "‏ إِنَّمَا أَنَا بَشَرٌ مِثْلُكُمْ أَنْسَى كَمَا تَنْسَوْنَ فَإِذَا نَسِيَ أَحَدُكُمْ فَلْيَسْجُدْ سَجْدَتَيْنِ وَهُوَ جَالِسٌ ‏" ‏ ‏.‏ ثُمَّ تَحَوَّلَ رَسُولُ اللَّهِ صلى الله عليه وسلم فَسَجَدَ سَجْدَتَيْنِ ‏.‏