Musnad Ahmad 26:55

Jarir narrated to us, from Mansur, from Ibrahim, from Alqamah, from Abdullah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) performed a prayer, and I do not know whether he added or subtracted. When he performed the taslim, it was said to him: "O Messenger of Allah, has something happened in the prayer?" He said: "No, and what is that?" They said: "You prayed such and such." He said: So he folded his legs and performed two prostrations of forgetfulness. When he performed the taslim, he said: "I am only a human being; I forget just as you forget. If one of you is in doubt during the prayer, let him seek what is correct regarding the prayer, then when he performs the taslim, let him perform two prostrations."