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."
Hadith ReferenceSahih Bukhari / The Book of Adhan (Sufa-tus-Salat) / Hadith: 780
Hadith Gradingمحدثین:أحاديث صحيح البخاريّ كلّها صحيحة