Sahih Al Bukhari – Book of Medicine
Volumn 007, Book 071, Hadith Number 619.
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Narated By Nazi' : Abdullah bin 'Umar said, "The Prophet said, 'Fever is from the heat of Hell, so put it out (cool it) with water.' " Nafi' added: 'Abdullah used to say, "O Allah! Relieve us from the punishment," (when he suffered from fever).