سنن النسائي

It was narrated from 'Abdullah that: The Messenger of Allah (ﷺ) prayed Zuhr then he turned to face them and they said: 'Has there been some change concerning the prayer?' He said: 'Why are you asking?' They told him what he had done, so he turned back toward the Qiblah and prostrated twice. Then he said the salam and turned to face them and said: 'I am only human, I forget as you forget, so if I forget, then remind me.' And he said: 'If there had been some change concerning the prayer I would have told you.' And he said: 'If one of you is not sure about his prayer, let him estimate what is closest to what is correct, then let him complete it on that basis, then prostrate twice.'