Sahih Muslim Book 26, Hadith Number 5482.
Chapter : There is a remedy for every malady and its excellence to get treatment.
Asma’ reported that a woman running high fever was brought to her. She asked water to be brought and then sprinkled it in the opening of a shirt at the uppermost part of the chest and said that Allah’s Messenger (may peace be upon him) had said: Cool (the fever) with water for it is because of the vehemence of the beat of Hell.