It is narrated from Hazrat Abu Huraira (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: When the Imam says "ghayril maghdubi 'alayhim wa la-d-dallin," the follower should say Ameen upon it, because whoever's Ameen coincides with the Ameen of the angels, all his past sins will be forgiven.