Bismillah Walhamdulillah Was Salaatu Was Salaam ‘ala Rasulillah
Narrated ‘Abdullah (Radi-Allahu ‘anhu):

I visited the Prophet during his ailments and he was suffering from a high fever. I said, “You have a high fever. Is it because you will have a double reward for it?” He said, “Yes, for no Muslim is afflicted with any harm but that Allah will remove his sins as the leaves of a tree fall down.”

Bukhari Vol. 7 : No. 550