Narrated Abu Huraira: The Prophet 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."
Hadith Referenceصحيح البخاري / كتاب الدعوات / 6402
Hadith Gradingمحدثین:أحاديث صحيح البخاريّ كلّها صحيحة