وَحَدَّثَنِي مَالِكٌ عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، أَنَّ رَسُولَ اللَّهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ قَالَ: «الْحُمَّى مِنْ فَيْحِ جَهَنَّمَ، فَأَطْفِئُوهَا بِالْمَاءِ»
It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that indeed the Messenger of Allah (peace and blessings be upon him) said: "Fever is from the boiling (steam and spreading of heat) of Hell, so extinguish it with water."