Sayyiduna Abu Hurairah, may Allah be pleased with him, narrates this about the Noble Prophet, peace and blessings be upon him. The Noble Prophet, peace and blessings be upon him, said: "When the Imam says «غير المغضوب عليهم ولا الضالین» , then you should say Ameen, because the angels also say Ameen and the Imam also says Ameen. Whoever's saying Ameen coincides with the angels' saying Ameen, his past sins are forgiven." (Imam Ibn Hibban, may Allah have mercy on him, says:) The statement of the Noble Prophet, peace and blessings be upon him, "So whoever's saying Ameen coincides with the angels' saying Ameen," means that the angels say Ameen without any cause such as showing off, seeking fame, self-admiration, etc. Their saying Ameen is purely for the sake of attaining the pleasure of Allah Almighty. So when the reciter says Ameen for the sake of attaining Allah Almighty's pleasure, without any cause such as self-admiration, showing off, seeking fame, etc., then in terms of sincerity, his saying Ameen will coincide with the angels' saying Ameen. In this case, that person's past sins will be forgiven.