Musnad Ahmad 4006

It was narrated that 'Abdullah said: The Messenger of Allah (ﷺ) led us in prayer, and he either added or omitted something. Ibraheem said: I am the one who forgot that. We said: O Messenger of Allah, has something been introduced into the prayer? He said: “Why is that?" We said: You just prayed such and such (number of rak'ahs). He said: l am only human and I forget as you forget. If one of you forgets something, let him prostrate twice.” Then he turned around and prostrated twice.