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 the imam says: 'Not of those who have earned (Your) anger, nor of those who go astray' [Surah al-Fatihah: 7], and those following him say Amin, and whoever's Amin coincides with the Amin of the inhabitants of the heaven (the angels), his past sins will be forgiven."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2436