(18) باب التَّسْمِيعِ وَالتَّحْمِيدِ وَالتَّأْمِينِ
Abu Huraira reported:
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.
Reference: Sahih Muslim 410 c
In-book reference: Book 4, Hadith 79
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 813