From Sahih Bukhari (Hadith 4475):
Narrated Abu Huraira:Allah's Messenger (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
Reference: Bukhari 4475