Abdullah bin ‘Umar (radiyallahu ta’ala ‘anhu) said, the Prophet (salallahu ‘alahi wasallam) said, ‘Fever is from the heat of Hell. Put it out (cool it) with water.’
(Bukhari)

Fatima (radiyallahu ta’ala ‘anha) narrated that Whenever a lady suffering from fever was brought to Asma’ bint Abu Bakr (radiyallahu ta’ala ‘anha) she used to invoke Allah for her and then sprinkle some water on her body at the chest and say, “Allah’s Messenger (salallahu ‘alahi wasallam) used to order us to abate fever with water”
(Bukhari)

Jabir ibn Abdullah (radiyallahu ta’ala ‘anhu) is reported to have related that the Messenger of Allah (salallahu ‘alahi wasallam) said. ‘Do not insult the fever, for it removes the sins of the son of Adam as the bellows remove the dross from iron.’
(Muslim)