Musnad Ahmad 27:1633

Yunus narrated to us, Hammad—meaning Ibn Salamah—narrated to us, from Bishr ibn Harb, who said: I heard Ibn Umar saying: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "Verily, for every treacherous person there is a banner by which he will be known according to the extent of his treachery. And verily, the greatest of treachery is the treachery of a general leader."