Mishkat al-Masabih 3737

Mu’adh b. Jabal told that when God’s Messenger sent him to the Yemen he asked him how he would judge when the occasion arose, and he replied that he would judge in accordance with God’s Book. He asked what he would do if he could not find guidance in God’s Book, and he replied that he would act in accordance with God’s Messenger’s sunnah. He asked what he would do if he could find no guidance in God’s Messenger’s sunnah, and he replied that he would do his best to form an opinion and spare no pains. God’s Messenger then tapped him on the breast and said, “Praise be to God who has disposed His Messenger’s Messenger to something with which God’s Messenger is pleased!”
Tirmidhi, Abu Dawud and Darimi transmitted it.