Musannaf Ibn Abi Shayba 35:54

Jarir narrated to us, from Mansur, from Ibrahim, from Alqama, from Abdullah, who said: The Messenger of Allah (may Allah bless him and grant him peace) performed a prayer and either added or omitted something. When he finished and turned his face toward the people, they said: "O Messenger of Allah, has something new occurred in the prayer?" He said: "And what is that?" They said: "You prayed such and such." So he folded his leg and performed two prostrations, then he performed the taslim and turned his face toward the people. He said: "If something new had occurred in the prayer, I would have informed you of it. But I am a human being; I forget just as you forget. So if I forget, remind me. And if one of you is in doubt regarding his prayer, let him seek what is correct, then complete it, then perform the taslim, then perform two prostrations."