(2) باب شِدَّةِ الْمَرَضِ
Narrated `Abdullah:
I visited the Prophet (ﷺ) 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."
Reference: Sahih al-Bukhari 5647
In-book reference: Book 75, Hadith 7
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 7, Book 70, Hadith 550