Musnad Ahmad 27:1723

Ya'qub narrated to us, 'Asim ibn Muhammad narrated to us, from his brother 'Umar ibn Muhammad, from Muhammad ibn Zayd, or Salim, from 'Abdullah ibn 'Umar, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Verily, fever is something from the scorching heat of Hell, so cool it with water."