Sayyiduna Abu Hurairah (may Allah be pleased with him) narrates that the Noble Prophet (peace and blessings be upon him) said: When the reciter (imam) says Ameen, you should also say Ameen, because the angels also say Ameen. Whoever’s Ameen coincides with the Ameen of the angels, all his previous sins will be forgiven.
Hadith Referenceالمنتقى ابن الجارود / كتاب الصلاة / 190