Sahih Bukhari Volume 7, Book 70, Hadith Number 551.
Narrated By ‘Abdullah : I visited Allah’s Apostle while he was suffering from a high fever. I said, “O Allah’s Apostle! You have a high fever.” He said, “Yes, I have as much fever as two men of you.” I said, “Is it because you will have a double reward?” He said, “Yes, it is so. No Muslim is afflicted with any harm, even if it were the prick of a thorn, but that Allah expiates his sins because of that, as a tree sheds its leaves.”