Characteristics of Prayer

Bukhari :: Book 1 :: Volume 12 :: Hadith 747

Narrated Abu Huraira:

The Prophet said, "Say Amin" when the Imam says it and if the Amin of any one of you coincides with that of the angels then all his past sins will be forgiven." Ibn Shihab said, "Allah's Apostle used to Say "Amin."