Sahih Bukhari Volume 007, Book 070, Hadith Number 550.

Narrated By 'Abdullah : I visited the Prophet during his ailments and he was suffering from a high fever. I said, "You have a high fever. Is it because you will have a double reward for it?" He said, "Yes, for no Muslim is afflicted with any harm but that Allah will remove his sins as the leaves of a tree fall down."