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."