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Sahih Muslim, Volume 1 The book of Prayer (Al-Salat)

Sahih Muslim, Vol. 1, Book II, Chapter CLXVI, P.225

(813) Abu Huraira reported: The Messenger of Allah, peace be upon him, said: When anyone amongst you utters Ameen in prayer and the angels in the sky also utter Ameen, and this (utterance of the one) synchronizes with (that of) the other, all his previous sins are pardoned.