Narrated Abu Huraira:
Allah's Apostle ﷺ said, "When the Imam says: 'Ghair-il-Maghdubi 'Alaihim Walad-Dallin (i.e. not the path of those who earn Your Anger, nor the path of those who went astray (1.7)), then you must say, 'Ameen', for if one's utterance of 'Ameen' coincides with that of the angels, then his past sins will be forgiven."
Sahih al-Bukhari, Vol. 6, Book of Prophetic Commentary on the Qur'an, Hadith 2
Sahih al-Bukhari, Book of Prophetic Commentary on the Qur'an, Hadith 2