The Messenger of Allah (ﷺ) led us five (rak'ahs in prayer). We said: Messenger of Allah, has the prayer been extended? He said: What is the matter? They said: You have said five (rak'ahs). He (the Holy Prophet) said: Verily I am a human being like you. I remember as you remember and I forget just as you forget. He then performed two prostrations as (compensation of) forgetfulness.
Sahih Muslim Book 5, Hadith 122
← Previous Hadith 'Alqama led us in the noon prayer and be offered five rak'ahs; when the prayer was complete, the people said to him: Abu Shibl, you have offered five rak'ahs. He said: No, I have not done that. They said: Yes (you said five rak'ahs). He (the narrator) said: And I was sitting in a corner among people and I was just a boy. I (also) said: Yes, you have offered five (rak'ahs). He said to me: O, one-eyed, do you say the same thing? I said: Yes. Upon this he turned (his face) and performed two prostrations and then gave salutations, and then reported 'Abdullah as saying: The Messenger of Allah (ﷺ) led us in prayer and offered five rak'ahs. And as he turned away the people began to whisper amongst themselves. He (the Holy Prophet) said: What is the matter with you? They said: Has the prayer been extended? He said: No. They said: You have in fact said five rak'ahs. He (the Holy Prophet) then turned his back (and faced the Qibla) and performed two prostrations and then gave salutations and further said: Verily I am a human being like you, I forget just as you forget. Ibn Numair made this addition:" When any one of you forgets, he must perform two prostrations."

Sahih Muslim Book 5, Hadith 121 Next Hadith → 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.

Sahih Muslim Book 5, Hadith 123