It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that he (peace and blessings be upon him) said: "When the imam says Amin, then you too say Amin, for the angels say Amin. Whoever's Amin coincides with the Amin of the angels, his past sins will be forgiven."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2434
Hadith Gradingمحدثین:صحيح
Hadith Takhrij«صحيح۔ تقدم في الذي قبله، وهو في البخاري 6402»