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," then you should say Ameen upon it, because the angels also say Ameen upon it and the Imam also says Ameen. So, whoever's Ameen coincides with the Ameen of the angels, all his past sins will be forgiven.