I heard the Prophet ﷺ saying, "Fever is from the heat of the (Hell) Fire; so cool it with water." (Using translation from Bukhārī 3262)   

سَمِعْتُ رَسُولَ اللهِ ﷺ يَقُولُ «الْحُمَّى مِنْ فَوْرِ جَهَنَّمَ فَأَبْرِدُوهَا بِالْمَاءِ»  

[Machine] The Messenger of Allah ﷺ said: "7564 and Al-Harith bin Miskeen recited it on him, and I heard from Ibn Al-Qasim who said: Malik narrated to us from Nafi', and Is'haq bin Ibrahim informed us, saying: Al-Mu'tamir and Muhammad bin Bishr narrated to us from Ubaidullah, from Nafi', from Abdullah bin Umar, from the Messenger of Allah ﷺ . And Al-Harith said from the Prophet ﷺ : 'Fever is a breath from the Hellfire, so cool it down with water.' Ibn Bishr said, 'The intensity of the fever.'"  

قَالَ رَسُولُ اللهِ ﷺ 7564 وَالْحَارِثُ بْنُ مِسْكِينٍ قِرَاءَةً عَلَيْهِ وَأنا أَسْمَعُ عَنِ ابْنِ الْقَاسِمِ قَالَ حَدَّثَنِي مَالِكٌ عَنْ نَافِعٍ وَأَخْبَرَنَا إِسْحَاقُ بْنُ إِبْرَاهِيمَ قَالَ أَخْبَرَنَا الْمُعْتَمِرُ وَمُحَمَّدُ بْنُ بِشْرٍ عَنْ عُبَيْدِ اللهِ عَنْ نَافِعٍ عَنِ ابْنِ عُمَرَ عَنْ رَسُولِ اللهِ ﷺ وَقَالَ الْحَارِثُ عَنِ النَّبِيِّ ﷺ قَالَ «إِنَّ الْحُمَّى مِنْ فَيْحِ جَهَنَّمَ فَأَبْرِدُوهَا بِالْمَاءِ» قَالَ ابْنُ بِشْرٍ شِدَّةُ الْحُمَّى