تبريد الحمى بالماء
Cooling the body with water during fever, based on the description of fever as a form of heat to be countered with cold.
الْحُمَّى مِنْ فَيْحِ جَهَنَّمَ فَأَبْرِدُوهَا بِالْمَاءِ
“Fever is from the heat of Hell, so cool it down with water.” (meaning, not a literal translation)
Ibn al-Qayyim explains this as cooling by cold water applied externally (poured or as a compress) rather than necessarily drinking large volumes, aimed at balancing the excess internal heat associated with fever.