Sahih Bukhari: Hadith 5446

I visited Allah's Messenger (ﷺ) while he was suffering from a high fever. I said, "O Allah's Messenger (ﷺ)! 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."