It is narrated from Sayyiduna 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, for whoever's Ameen coincides with the Ameen of the angels, his previous sins are forgiven." Ibn Shihab (az-Zuhri, may Allah have mercy on him) said: The Messenger of Allah (peace and blessings be upon him) used to say Ameen.

And with the same chain of narration, it is narrated from (Sayyiduna Abu Hurairah, 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. Then if the Ameen of both coincides, the person's previous sins are forgiven."

And with the same chain of narration, it is narrated from (Sayyiduna Abu Huraira, may Allah be pleased with him) that the Messenger of Allah, peace and blessings be upon him, said: "When the Imam says «غَيْرِ الْمَغْضُوبِ عَلَيْهِمْ وَلَا الضَّالِّينَ» , then you should say Ameen, for whoever's saying coincides with the saying of the angels, his previous sins are forgiven."