Musannaf Ibn Abi Shaybah 4465 — A man who prays and does not know

Sayyiduna Abdullah (bin Mas'ud) (may Allah be pleased with him) said: Once the Prophet (peace and blessings be upon him) led a prayer and added something to it or left something out. When he gave the salam and turned to face the people, they said, "O Messenger of Allah, has some new command been revealed about the prayer?" He asked, "Why, what has happened?" The people said, "Today you prayed such-and-such a prayer." The Prophet (peace and blessings be upon him) at once turned his feet and made two prostrations. Then he gave the salam, turned to face the people, and said: "If any command about the prayer had been revealed, I would certainly have told you. Rather, the truth is that I am a human being like you; just as you forget, I too can forget. When I forget in the prayer, remind me. When any of you makes a mistake in his prayer, let him reflect and act on what seems correct to him. Then, when he gives the salam, let him make two prostrations."