Musannaf Ibn Abi Shayba 20:59

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Shu’bah, from Abu ‘Awn, from al-Harith ibn ‘Amr al-Hudhali, from a man from the people of Homs, from the companions of Mu’adh, from Mu’adh: That when the Prophet, peace and blessings of Allah be upon him, sent him, he said: "How will you judge?" He said: "I will judge by the Book of Allah." He said: "If it is not in the Book?" He said: "I will judge by the Sunnah of the Messenger of Allah, peace and blessings of Allah be upon him." He said: "If it is not in the Sunnah of the Messenger of Allah, peace and blessings of Allah be upon him?" He said: "I will exert my opinion." He said: Then the Prophet, peace and blessings of Allah be upon him, said: "Praise be to Allah who granted success to the messenger of the Messenger of Allah."