It is narrated from Sayyiduna Abu Huraira (may Allah be pleased with him) that the Prophet of Allah (peace and blessings be upon him) said: "When the Imam says «غَيْرِ الْمَغْضُوبِ عَلَيْهِمْ وَلَا الضَّالِّينَ» , then you should say Ameen, because the angels also say Ameen and the Imam also says Ameen. So, whoever's saying Ameen coincides with the angels' saying Ameen, all his previous sins will be forgiven."