It is narrated from Abu Hurairah (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 sins are forgiven." Ibn Shihab Zuhri (may Allah have mercy on him) says that the Messenger of Allah (peace and blessings be upon him) used to say Ameen.
Hadith Referenceمسند الإمام الشافعي / كتاب الصلاة / 216
Hadith Takhrijأخرجہ البخاری، الاذان، باب جہر الامام بالتأمین: 780 - ومسلم، الصلاۃ، باب التسمیع والتحمید والتأمین: 401۔