Sahih Muslim Book 4, Hadith Number 813.

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: When anyone amongst you utters Amin in prayer and the angels in the sky also utter Amin, and this (utterance of the one) synchronises with (that of) the other, all his previous sins are pardoned.