From Sahih Bukhari (Hadith 6402):
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: Bukhari 6402