Musnad Ahmad 26:797

Muhammad ibn Ubayd narrated to us, Mis'ar 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) said: "I am only a human being; I forget just as you forget. So whoever among you is in doubt regarding his prayer, let him look for what is most likely to be correct, complete it accordingly, and perform two prostrations."