Sahih Bukhari Volume 007, Book 070, Hadith Number 565.
Narrated By 'Abdullah : I visited the Prophet during his illness and touched him while he was having a fever. I said to him, "You have a high fever; is it because you will get a double reward?" He said, "Yes. No Muslim is afflicted with any harm, but that his sins will be annulled as the leave of a tree fall down."