Sahih Bukhari Volume 7, Book 70, Hadith Number 571.

Narrated By Ibn Mas’ud : I visited the Prophet while he was having a high fever. I touched him and said, “You have a very high fever” He said, “Yes, as much fever as two me of you may have.” I said. “you will have a double reward?” He said, “Yes No Muslim is afflicted with hurt caused by disease or some other inconvenience, but that Allah will remove his sins as a tree sheds its leaves.”