Musnad Ahmad 27:1127

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from 'Umar ibn Muhammad ibn Zayd, that he heard his father narrating from Ibn 'Umar, from the Prophet, peace and blessings of Allah be upon him, that he said: "Fever is from the scorching heat of Hell, so extinguish it with water, or cool it with water."