The Prophet ﷺ said, "Fever comes from the heat of Hell, so cool it down with water," or, "extinguish it with water."
حدثنا محمد بن جعفر حدثنا شعبة عن عمر بن محمد بن زيد أنه سمع أباه يحدث
عن النبي ﷺ أنه قال الحمى من فيح جهنم فأطفئوها بالماء أو بردوها بالماء
Its isnad is sahih. Muslim related it 2:185 through the two routes of Muhammad bin Jafar and Rawh, both from Shubah, with this isnad. It has already passed through the route of Ubaydullah from Nafi from Ibn Umar, 4719.Ibn Athir said, "Fayh means the blazing and surging of heat, and it is also said with a waw. It is said, 'The pot faaha, yafeeh and yafuuh,' when it boils." It was presented in the form of an analogy and comparison.