Sahih Bukhari Volume 008, Book 075, Hadith Number 411.

Narated By 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."