Sahih Muslim
The Book of Jihad and Expedition (Kitab Al-Jihad wal-Siyar)
Book Number: 019 / Hadith Number: 4310
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.