Sahih Muslim Book 4, Hadith Number 816.

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 the reciter (Imam) utters: “Not of those on whom (is Thine) wrath and not the erring ones,” and (the person) behind him utters Amin and his utterance synchronises with that of the dwellers of heavens, all his previous sins would be pardoned.