Musannaf Ibn Abi Shayba 13:3030

Abu Bakr narrated to us, he said: Abu Mu'awiyah narrated to us, from al-Shaybani, from Muhammad ibn Abd Allah al-Thaqafi, he said: When the Messenger of Allah (may Allah bless him and grant him peace) sent Mu'adh to Yemen, he said: "O Mu'adh, by what will you judge?" He said: "I will judge by the Book of Allah." He said: "If a matter comes to you that is not in the Book of Allah, and the Prophet (may Allah bless him and grant him peace) has not judged upon it, and the righteous have not judged upon it?" He said: "I will exert my effort to reach the truth." He said: So the Messenger of Allah (may Allah bless him and grant him peace) said: "Praise be to Allah who has made the messenger of the Messenger of Allah (may Allah bless him and grant him peace) judge by that which pleases the Messenger of Allah (may Allah bless him and grant him peace)."