Abu Hurairah reported the Messenger of Allah (May peace be upon him) as saying; When the Imam says Amin, say Amin, for if anyone’s utterance of Amin synchronnises with that of the angles, he will be forgiven his past sins. Ibn shihab (al Zuhrl) said; The Messenger of Allah (May peace be upon him) used to say Amin (At the end of the Fatihah)

English translation · Sunan Abu Dawud