Yahya related to me from Malik from Hisham ibn Urwa from hisfather that the Messenger of Allah, may Allah bless him and grant himpeace, said, "Fever is from the vehemence of the heat of Jahannam, socool it with water."
USC-MSA web (English) reference: Book 50, Hadith 16
Arabic reference: Book 50, Hadith 1729