Sayyiduna Abu Hurairah (may Allah be pleased with him) narrates that I heard the Messenger of Allah (peace and blessings be upon him) say: "When the Imam says Ameen, you should also say Ameen. Thus, whoever's Ameen coincides with the Ameen of the angels, his previous sins are forgiven."
When the imam says "Ameen," the believer hopes for forgiveness of his past sins if his saying "Ameen" coincides with that of the angels, with evidence that the imam is required to say "Ameen" aloud when he recites aloud, so that the follower may hear his "Ameen." For it is not permissible for the Prophet, peace and blessings be upon him, to command the follower to say "Ameen" when his imam says "Ameen," while there is no way for him to know the imam's "Ameen" if the imam conceals it.
Hadith Referenceصحيح ابن خزيمه / جماع أبواب قيام المأمومين خلف الإمام وما فيه من السنن / 1583