The Prophet ﷺ said, "Fever, or the intensity of fever, is from the blast of Hellfire, so cool it down with water."
حدثنا يحيى عن هشام قال أخبرني أبي
عن النبي ﷺ إن الحمى أو شدة الحمى من فيح جهنم فأبردوها بالماء
Its isnad is sahih, Bukhari narrated it (6/330, no. 3264) in the Beginning of Creation, Muslim (4/1732, no. 2210) in Greetings, Tirmidhi (4/404, no. 2074) and graded it sahih, and Ibn Majah (2/1149, no. 3471), both of them in Medicine.