It is narrated from Abu Huraira (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When any one of you says Ameen, the angels in the heavens also say Ameen, and whoever's Ameen coincides with the Ameen of the angels, all his previous (minor) sins are forgiven."
Hadith Referenceمسند الإمام الشافعي / كتاب الصلاة / 215
Hadith Takhrijاخرجه البخاری: الاذان، باب فضل التامین: 781 - ومسلم، الصلاة، باب التسمیع والتحمید والتامین: 410۔