Narrated by Abu Huraira (RA): 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." [1] Narrated by Abu Huraira (RA): Allah's Messenger (ﷺ) said, "Say Amen" when the Imam says 'Ghairil maghdubi 'alaihim walad daalleen' (not the path of those who earn Your Anger (such as Jews) nor of those who go astray (such as Christians); all the past sins of the person whose saying (Amin) coincides with that of the angels, will be forgiven". [2]

Reference: [1] Bukhari: 780 [2] Bukhari: 782