Sahih Muslim Book 032, Hadith Number 6244.
Chapter : Whenever a believer falls sick or is stricken with grief or something tike it, there is a reward for him even if it is pricking of a thorn.
Jabir b. Abdullah reported that Allah's Messenger (may peace be upon him) visited Umm Sa'ib or Umm Musayyib and said: Umm Sa'ib or Umm Musayyib. why is it that you are shivering? She said: "It is fever and may it not be blessed by Allah, whereupon he (the Holy Prophet) said: Don't curse fever for it expiates the sir of the posterity of Adam just as furnace removes the alloy of iron.