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 «غَيْرِالْمَغْضُوبِعَلَيْهِمْوَلَاالضَّالِّينَ», 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."
Hadith Referenceسنن دارمي / من كتاب الصللاة / 1280
From these two hadiths, the virtue of saying "Ameen" is understood. Also, it is known that the angels also say "Ameen," and the imam also says "Ameen." Therefore, the followers (muqtadis) should also say "Ameen" aloud, so that it becomes an expiation for sins. Another evidence for this is coming ahead.