Fever is from the searing heat of Hell, so cool it off with water
Authentic hadithعن عائشة رضي الله عنها أن النبي صلى الله عليه وسلم قال: «الحُمَّى من فَيْحِ جهنم فأبردوها بالماء».
‘Ā'ishah (may Allah be pleased with her) reported the Prophet (may Allah's peace and blessings be upon him) said: "Fever is from the searing heat of Hell, so cool it off with water."
Explanation
The Prophet (may Allah's peace and blessings be upon him) informed us that fever is from the intense heat of Hellfire. So, the heat that the sick person experiences is from it. Therefore, he urged us to overcome such heat by water.
Source · Narrated by Bukhari & Muslim
From the Hadith Encyclopedia (hadeethenc.com).