The Book of Prayers (Kitab Al-Salat)

Muslim :: Book 4 : Hadith 1179

'Abdullah (b. Mas'ud) reported: The Messenger of Allah (may peace be upon him) 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 (may peace be upon him) turned (his face towards the Qibla) and performed two prostrations.

Source materials are from the University of Southern California MSA site
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