Musnad Abu Ya'la al-Mawsili 152:74

Abu Khaythama narrated to us, Jarir narrated to us, Mansur narrated to us, from Ibrahim, from Alqama, who said: Abdullah said: The Messenger of Allah (ﷺ) performed a prayer. Ibrahim said: I do not know whether he added or decreased. When he performed the taslim, it was said to him: "O Messenger of Allah, has something new occurred in the prayer?" He said: "No, and what is that?" They said: "You prayed such and such." He said: So he folded his leg and faced the Qibla and prostrated two prostrations, then he performed the taslim. When he performed the taslim, he turned his face toward us and said: "If something new had occurred in the prayer, I would have informed you of it. But I am only a human being like you; I forget just as you forget. So if I forget, remind me. And if one of you is in doubt during the prayer, let him seek what is correct, then complete it upon that, then perform the taslim, then prostrate two prostrations."