The Prophet ﷺ said, "Fever is from the blazing heat of Hell, so cool it with water."
عن النبي ﷺ قال: الحمى من فيح جهنم فأبردوها بالماء.
Bukhari narrated it in al-Tibb (5725), and Muslim in al-Salam (2210), both through the route of Hisham bin Urwa, from his father, from Ayesha, who narrated it.