Allah's Apostle said, "Say Amen' when the Imam says "Ghair-il-maghdubi 'alaihim wala-ddal-lin; not the path of those who earn Your Anger (such as Jews) nor of those who go astray (such as Christians); all the past sins of the person whose saying (of Amin) coincides with that of the angels, will be forgiven.

Narrated Abu Huraira, in Bukhari, Volume 1, Book 12, Number 749 - Sunni Hadith