Musnad Ahmad 4147

It was narrated that ‘Abdullah said: The Messenger of Allah (ﷺ) offered the prayer and I do not know whether he added or omitted something. Then he turned to face us and we told him what he had done, so he turned to face the qiblah and prostrated twice, then he turned to farc us and said: "If something had been introduced into the prayer, I would have told you. But I am only human and I forget as you forget, so if I forget then remind me. And if any of you has any doubts concerning his prayer, let him think of what is most likely to be the case and then complete (his prayer on that basis) and say the salam, then prostrate twice."