Sahih Muslim Book 4, Hadith 81
'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).
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 the reciter (Imam) utters:" Not of those on whom (is Thine) wrath and not the erring ones," and (the person) behind him utters Amin and his utterance synchronises with that of the dwellers of heavens, all his previous sins would be pardoned.