Sahih Muslim Book 4, Hadith 78
I heard from the Messenger of Allah (ﷺ) the hadith like one transmitted by Malik, but he made no mention of the words of Shihab.
The Messenger of Allah (ﷺ) said: SayAmin when the Imam says Amin, for it anyone's utterance of Amin synchronises with that of the angels, he will be forgiven his past sins.
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.