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], then say Amin, for whoever's Amin coincides with the Amin of the angels, his past sins will be forgiven."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2435