Musannaf Ibn Abi Shaybah

It is narrated from Hazrat Abdullah that the Messenger of Allah, peace and blessings be upon him, led a prayer in which he either omitted or added something. When he turned his blessed face towards the people after saying salam, the people said, "O Messenger of Allah (peace and blessings be upon him), has something new occurred in the prayer?" The Prophet (peace and blessings be upon him) said, "What happened?" The people said, "You led the prayer in such a manner (with omission or addition)." The Prophet (peace and blessings be upon him) folded his legs and performed two prostrations, then after saying salam, he turned his blessed face towards the people and said, "If something new had occurred in the prayer, I would have informed you of it. But I am a human being; I forget just as you do. So when I forget, remind me. And if any one of you is uncertain about his prayer, he should try to determine what is correct, then complete his prayer based on that determination. Then, after saying salam, he should perform two prostrations."

It is narrated from Hazrat Abdullah that the Noble Prophet (peace and blessings be upon him) once led the Zuhr prayer with five rak‘ahs. It was said to him, "Did you pray five rak‘ahs?" So after the salam, the Prophet (peace and blessings be upon him) performed two prostrations. And (Imam) Abu Hanifa's opinion is mentioned that: If one does not sit for the tashahhud in the fourth rak‘ah, he must repeat the prayer.