Sahih Muslim
The Book of Jihad and Expedition (Kitab Al-Jihad wal-Siyar)
Book Number: 019 / Hadith Number: 4309
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 fixed behind the buttocks of every person guilty of the breach of faith.