Musnad Ahmad 31:675

Hassan ibn Musa narrated to us, Hammad ibn Zayd narrated to us, from Ali ibn Zayd, from Abu Nadra, from Abu Sa’id al-Khudri, from the Prophet, peace and blessings of Allah be upon him, who said: "Behold, for every treacherous person there shall be a banner on the Day of Resurrection, in proportion to his treachery. Behold, and there is no treachery greater than that of a leader of the public."