I heard Allah's messenger (ﷺ) as saying: The fever is due to the intense heat of the Hell, so cool it with water.
Sahih Muslim Book 39, Hadith 113
← Previous Hadith " She poured water on her sides and in the opening of the shirt at the uppermost part of the chest." There is no mention of these words:" It is from the vehemence of the heat of the Hell." This hadith has been narrated on the authority of Abu Usama with the same chain of transmitters.

Sahih Muslim Book 39, Hadith 112 Next Hadith → I heard Allah's Messenger (ﷺ) 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.

Sahih Muslim Book 39, Hadith 114