(63) باب التَّأْمِينِ
Narrated Abu Huraira:
The Prophet (ﷺ) said, "When the Imam says 'Amin', then you should all say 'Amin', for the angels say 'Amin' at that time, and he whose 'Amin' coincides with the 'Amin' of the angels, all his past sins will be forgiven."
Reference: Sahih al-Bukhari 6402
In-book reference: Book 80, Hadith 97
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 8, Book 75, Hadith 411