Sayyiduna Abu Hurairah (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When the reciter says «غَيْرِالْمَغْضُوبِعَلَيْهِمْوَلَاالضَّالِّينَ», then those who are behind him (the followers in prayer) should say Ameen, and if this Ameen coincides with that of those in the heavens, all his previous sins will be forgiven."
Hadith Referenceسنن دارمي / من كتاب الصللاة / 1279
Hadith Gradingتحقیق (حسین سلیم أسد الدارانی):إسناده حسن من أجل محمد بن عمرو ولكن الحديث صحيح، [مكتبه الشامله نمبر: 1281]
Hadith Takhrijاس سند سے یہ حدیث حسن ہے، لیکن دوسری سند سے صحیح متفق علیہ ہے۔ دیکھئے: [بخاري 782] ، [مسلم 410] ، [مسند أبى يعلی 5874] ، [ابن حبان 1804]
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