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 : Book 80, Hadith 97 USC-MSA web (English) : Vol. 8, Book 75, Hadith 411 (deprecated)
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