Sahih Muslim Book 19, Hadith Number 4310.

Chapter : Prohibition (denunciation) of branch of faith.

It is narrated on the authority of Abu Sa’id that the Messenger of Allah (may peace be upon him) said: On the Day of Judgment there will be a flag for every person guilty of the breach of faith. It will be raised in proportion to the extent of his guilt; and there is no guilt of treachery more serious than the one committed by the ruler of men.