Narrated Abu Huraira (May Allah Subhanahu wa ta'ala be pleased with the him) Allah's Apostle (May Peace and blessings and mercy of Allah Subhanahu wa ta'ala be on him) 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 Bukhari:Volume 6, Book 60, Number 2