Fever is from the vehement raging of the Hell-fire, so cool it with water. (Using translation from Muslim 2210a)

سَمِعْتُ رَسُولَ اللهِ ﷺ يَقُولُ «الْحُمَّى مِنْ فَوْرِ جَهَنَّمَ فَأَبْرِدُوهَا بِالْمَاءِ»