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

"The Messenger of Allah (saw) offered prayer, and I am not sure whether he did something extra or omitted something. He asked, and we told him, so he turnd to face the Qiblah and prostrated twice, then he said the Salam. Then he turned to face us and said: 'If any new command has been revealed concerning the prayer, I would certainly have told you. But I am only human and I forget and you forget. If I forget, then remind me. And if anyone of you is uncertain about the prayer, let him do what is closest to what is correct, then complete the prayer, say the Salam and prostrate twice."