From Sahih Bukhari (Hadith 780):
Narrated Abu Huraira:The Prophet (ﷺ) 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 Messenger (ﷺ) used to Say "Amin
Reference: Bukhari 780