Musnad Ahmad 1018:95

Ishaq ibn Yusuf narrated to us, Sufyan narrated to us, from Mansur, from Hilal ibn Yasaf, from Dhakwan, from a man from the Ansar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) visited a man who had a wound. The Messenger of Allah (peace and blessings of Allah be upon him) said: "Call the physician of the Banu so-and-so for him." He said: So they called him and he came. They said: "O Messenger of Allah, does medicine avail anything?" He said: "Glory be to Allah, has Allah sent down any disease upon the earth, but He has created a cure for it?"