It is narrated from Sayyiduna Thawban (may Allah be pleased with him) that he said: The Messenger of Allah (peace and blessings be upon him) said: "When fever afflicts one of you, then indeed fever is a piece of the fire of Hell, so let him extinguish it with cold running water, facing the direction of the water's flow, and let him say: 'Allahumma saddiq rasulaka wa-shfi abdaka' (O Allah, prove Your Messenger truthful and heal Your servant), after Fajr and before sunrise, and let him immerse himself in it three times. If he is not cured in five days, then in seven; and if he is not cured in seven, then in nine, for by the permission of Allah it hardly ever goes beyond nine."
Hadith Referenceالمعجم الكبير للطبراني / باب الثاء / 1437