Sahih Muslim: Hadith 1277

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."