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." (Using translation from Bukhārī 6402)
"إذا أمَّن القارئُ فأَمِّنوا فإن الملائكة تؤمِّن- فمن وافقَ تأمينُه تأمينَ الملائكةِ غُفِرَ له ما تقدَّم من ذنْبِه".