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 the Imam says Ameen, you should also say Ameen! Whoever's Ameen coincides with the Ameen of the angels, all his previous (minor) sins are forgiven."
Hadith Referenceمسند الإمام الشافعي / كتاب الصلاة / 213
Hadith Takhrijأخرجہ البخاری، الاذان، باب جہر الامام بالتأمین: 780 - ومسلم، الصلاۃ، باب التسمیع والتحمید والتأمین: 401۔