Fever is from the heat of the Hell-Fire so cool it down with water
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا عَبْدَةُ بْنُ سُلَيْمَانَ، عَنْ هِشَامِ بْنِ عُرْوَةَ، عَنْ فَاطِمَةَ بِنْتِ الْمُنْذِرِ، عَنْ أَسْمَاءَ بِنْتِ أَبِي بَكْرٍ، أَنَّهَا كَانَتْ تُؤْتَى بِالْمَرْأَةِ الْمَوْعُوكَةِ فَتَدْعُو بِالْمَاءِ فَتَصُبُّهُ فِي جَيْبِهَا وَتَقُولُ إِنَّ النَّبِيَّ ـ صلى الله عليه وسلم ـ قَالَ " ابْرُدُوهَا بِالْمَاءِ " . وَقَالَ " إِنَّهَا مِنْ فَيْحِ جَهَنَّمَ " .
It was narrated from Asma’ bint Abu Bakr that a woman suffering from fever would be brought to her, and she would call for water and pour it onto the neck of her garment. She said: The Prophet (ﷺ) said: “Cool it down with water,” and he said: “It is from the heat of Hell-fire.”