Narrated 'Abdullah (may Allah swt be pleased with him):

I visited the Prophet (may peace and blessings of Allah swt shower on him) 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."

Bukhari :: Book 7 :: Volume 70 :: Hadith 550