Sayyiduna Abu Hurairah (may Allah be pleased with him) narrates from the Noble Prophet (peace and blessings of Allah be upon him) that he (peace be upon him) said: "When the reciter says Ameen, you should also say Ameen, because the angels also say Ameen. Therefore, whoever's Ameen coincides with the Ameen of the angels, his past sins will be forgiven."