Musannaf Ibn Abi Shayba 21:385

Abu Bakr narrated to us, 'Affan narrated to us, Hammad ibn Salamah narrated to us, from Humayd, from Anas, that the Messenger of Allah (may Allah bless him and grant him peace), when he would enter upon a sick person, would say: "Remove the hardship, O Lord of the people, and heal, for You are the Healer. There is no healer but You, a healing that leaves behind no illness."