Muwatta Malik · Hadith 1726
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ هِشَامِ بْنِ عُرْوَةَ، عَنْ أَبِيهِ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " إِنَّ الْحُمَّى مِنْ فَيْحِ جَهَنَّمَ فَابْرُدُوهَا بِالْمَاءِ " .
Yahya related to me from Malik from Hisham ibn Urwa from his father that the Messenger of Allah, may Allah bless him and grant him peace, said, "Fever is from the vehemence of the heat of Jahannam, so cool it with water
Sahih · Salim al-Hilali