Sahih Bukhari

Narrated `Abdullah: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