Sahih Muslim Book 32, Hadith Number 6223.
Chapter : It is forbidden to nurse rancour.
Abu Huraira reported it as a marfu’ hadith (and the words are): The deeds are presented on every Thursday and Friday and Allah, the Exalted and Glorious, grants pardon to every person who does not associate anything with Allah except the person in whose (heart) there is rancour against his brother. It would be said: Put both of them off until they are reconciled.