From Sahih Muslim (Hadith 1285):
Abdullah (b. Mas'ud) reported:The Messenger of Allah (ﷺ) 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 (ﷺ) turned (his face towards the Qibla) and performed two prostrations
Reference: Muslim 1285