Sahih Muslim Book 4, Hadith 80
The Messenger of Allah (ﷺ) said: When anyone amongst you utters Amin and the angels In the heaven also utter Amin and (the Amin) of the one synchronises with (that of) the other, all his previous sins are pardoned.
The Messenger of Allah (ﷺ) said: When anyone amongst you utters Amin in prayer and the angels in the sky also utter Amin, and this (utterance of the one) synchronises with (that of) the other, all his previous sins are pardoned.
'A hadith like this is transmitted by Ma'mar from Hammam b. Munabbih on the authority of Abu Huraira who reported it from the Messenger of Allah (ﷺ).