I heard Messenger of Allah ﷺ as saying: The fever is due to the intense heat of the Hell, so cool it with water. (Using translation from Muslim 2212a)
سَمِعْتُ رَسُولَ اللهِ ﷺ يَقُولُ إِنَّ الْحُمَّى فَوْرٌ من فَوْرِ جَهَنَّمَ فَابْرُدُوهَا بِالْمَاءِ