Musannaf Ibn Abi Shayba 21:764

Mu'awiyah ibn Hisham narrated to us, he said: Sufyan narrated to us, from 'Abd al-Malik ibn 'Umayr, from a man, from Makhul, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "There is no man who has a fever, then he bathes for three consecutive days, and says at every bath: 'In the name of Allah, O Allah, I have only bathed in hope of Your healing and in confirmation of Your Prophet Muhammad (peace and blessings of Allah be upon him),' but it will be removed from him."