It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When one of you says Amin, the angels in the heaven also say Amin. If the Amin of one of them coincides with that of the other, his past sins will be forgiven."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2437