(113) باب جَهْرِ الْمَأْمُومِ بِالتَّأْمِينِ
Narrated Abu Huraira:
Allah's Messenger (ﷺ) said, "Say Amen when the Imam says 'Ghairi l-maghdubi `alaihim wala d-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 (of Amin) coincides with that of the angels, will be forgiven".
Reference: Sahih al-Bukhari 782
In-book reference: Book 10, Hadith 177
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 1, Book 12, Hadith 749