Musnad Abu Ya'la al-Mawsili 42:324

Zuhayr narrated to us, Uthman ibn Umar narrated to us, al-Mustamir ibn al-Rayyan narrated to us, from Abu Nadra, from Abu Sa’id, from the Prophet (peace be upon him), who said: "Let not the fear of a man or the fear of people prevent any one of you from speaking the truth when he sees it or knows it." Abu Sa’id said: So I met Mu’awiyah and said to him: "There is no one who commits treachery except that he will have a banner of treachery on the Day of Resurrection for his treachery, and there is no traitor greater than a leader of the public."