Musnad Abu Ya'la al-Mawsili 245:27

Suwayd ibn Sa'id narrated to us, 'Amr ibn Yahya ibn Sa'id narrated to us, from his grandfather Sa'id ibn 'Amr ibn al-'As, from Mu'awiyah, who said: The Messenger of Allah (ﷺ) said: "Perform ablution." He said: When I performed ablution, he looked at me and said: "O Mu'awiyah, if you are put in charge of an affair, then fear Allah and be just." He said: I continued to think that I would be tested with a position of authority due to the statement of the Messenger of Allah (ﷺ), until I was put in charge.