Sahih Bukhari
Invocations
Volume: 008 / Book Number: 075 / Hadith Number: 411
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."