It is narrated from Hazrat Abu Huraira (may Allah be pleased with him) that the Holy Prophet (peace and blessings be upon him) said: When the reciter (the Imam) says Ameen, you should also say Ameen, because if a person's Ameen coincides with the Ameen of the angels, all his past sins will be forgiven.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 9921