Musnad Ahmad 5:4

Abu Sa'id, the freed slave of Banu Hashim, narrated to us, Isra'il narrated to us, Abu Ishaq narrated to us, from al-Harith, from Ali, who said: When the Messenger of Allah (peace and blessings of Allah be upon him) would seek refuge for a sick person, he would say: "Remove the harm, O Lord of the people, and heal, for You are the Healer. There is no healing but Your healing, a healing that leaves behind no illness."