From Sahih Bukhari (Hadith 5660):
Narrated `Abdullah bin Mas`ud:I visited Allah's Messenger (ﷺ) while he was suffering from a high fever. I touched him with my hand and said, "O Allah's Messenger (ﷺ)! You have a high fever." Allah's Messenger (ﷺ) said, "Yes, I have as much fever as two men of you have." I said, "Is it because you will get a double reward?" Allah's Messenger (ﷺ) said, "Yes, no Muslim is afflicted with harm because of sickness or some other inconvenience, but that Allah will remove his sins for him as a tree sheds its leaves
Reference: Bukhari 5660