Musnad Ahmad 945:1

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Habib al-Ansari, who said: I heard Ibn Abi Bashir and the daughter of Abi Bashir narrating from their father, from the Prophet, may Allah bless him and grant him peace, that he said regarding fever: "Cool it with water, for it is from the heat of Hell."