Sahih Muslim Book 26, Hadith Number 5485.

Chapter : There is a remedy for every malady and its excellence to get treatment.

Rafi’ b. Khadij reported: I heard Allah’s Messenger (may peace be upon him) as saying: The fever is due to the intense heat of Hell, so cool it down in your (bodies) with water. Aba Bakr has made no mention of the word “from you” (‘ankum), but he said that Rafi’ b. Khadij had informed him of it.