I heard the Prophet ﷺ say, "Fever is from the heat of Hell, so cool it down with water." He then visited a son of Ammar and said, "Remove the affliction, Lord of mankind, God of mankind."
حدثنا محمد بن عبد الله بن نمير حدثنا مصعب بن المقدام حدثنا إسرائيل عن سعيد بن مسروق عن عباية بن رفاعة
سمعت النبي ﷺ يقول الحمى من فيح جهنم فابردوها بالماء فدخل على ابن لعمار فقال اكشف الباس رب الناس إله الناس
Bukhari narrated it in the Book of Medicine, chapter "Fever is from the intensity of the heat of Hell," hadith no. 5726, and Muslim in the Book of Medicine, chapter "Every disease has a cure and the recommendation of seeking treatment," hadith no. 2212, from the hadith of Saeed.