Musnad Abu Ya'la al-Mawsili 129:77

Harun al-Hammal narrated to us, Rawh ibn 'Ubadah narrated to us, Hammad ibn Salamah narrated to us, Humayd informed us, from Anas, that the Messenger of Allah (may Allah bless him and grant him peace) said: "If one of you develops a fever, let him pour cold water upon himself for three nights at the time of the dawn (al-sahar)."