The Messenger of Allah ﷺ said, "Fever is from the blast of Hellfire, so cool it down with Zamzam water."
إن رسول الله ﷺ قال: إن الحمى من فيح جهنم، فأبردوها بماء زمزم.
Imam Ahmad narrated it (2649), and Ibn Hibban graded it sahih (6028), as did Hakim (4/403), all of them through the route of Affan, who said, Hammam narrated to us, Abu Jamra informed us, he said, I used to keep people away from Ibn Abbas. One day I was absent for several days, so he asked, "What kept you away?" I said, "Fever." He said, "The Messenger of Allah said..." and he mentioned the hadith.Hakim said, "It is sahih according to the criteria of the two Shaykhs, though they did not narrate it in this wording." I say, this is as he said, except that he was mistaken in claiming it as a supplement to Bukhari, because Bukhari did narrate this hadith (3261), from Abdullah bin Muhammad, who said, Abu Amir narrated to us, Hammam narrated to us, with his chain. In it, Ibn Abbas said, "Cool it down for yourself with Zamzam water," then he said, the Messenger of Allah ﷺ said, "Fever comes from the exhalation of Hell, so cool it with water," or he said, "with Zamzam water," Hammam being uncertain about it.So perhaps Hakim narrated it for the sake of the certainty regarding Zamzam water, since Bukhari did not narrate it in this wording, meaning with certainty. Affan is Ibn Muslim, a precise and meticulous imam and hafiz. Ibn Madini said, "If he was uncertain about a single word in a hadith, he would leave it out." So his certainty takes precedence over the one who was uncertain in narrating from Hammam, namely Abu Amir Aqadi (Abdul-Malik bin Amr Qaysi), through whose route Bukhari narrated, and who is below Affan bin Muslim in precision and mastery of memorization.The mention of Zamzam in this hadith does not preclude cooling a fever with ordinary water for someone who cannot find Zamzam water. This is because Bukhari, after narrating the hadith of Ibn Abbas and noting that he was in Makkah where Zamzam water is found, then narrated after it the hadith of Rafi bin Khadij, who said, I heard the Prophet ﷺ say, "Fever comes from the boiling heat of Hell, so cool it down with water." Likewise, he narrated the hadith of Ayesha and Ibn Umar, indicating the use of ordinary water for cooling a fever. So whoever finds Zamzam water may cool it with that, and whoever cannot find it may cool it with whatever water is available.