Sahih Muslim Book 4, Hadith Number 811.
Chapter : Blessing on the Prophet (may peace be upon him) after Tashahhud.
Abu Huraira reported: The Messenger of Allah (may peace be upon him) said: Say Amin when the Imam says Amin, for it anyone’s utterance of Amin synchronises with that of the angels, he will be forgiven his past sins.