Sunan Ibn Majah, Chapter: The Chapters of Establishing the Prayer and the Sunnah Regarding Them

"The Messenger of Allah (saw) prayed, and he added or omitted something." (One of the narrators) Ibrahim said: "The confusion stems from me (i.e., he was not sure which it was)." "It was said to him: 'O Messenger of Allah! Has something been added to the prayer?' He said: 'I am only human, I forget just as you forget. If anyone forgets, let him perform two prostrations when he is sitting (at the end).' Then the Prophet (saw) turned and prostrated twice." Hasan