Hadith – Bukhari 7:564
I visited Allah’s Apostle while he was suffering from a high fever. I touched him with my hand and said, “O Allah’s Apostle! You have a high fever.” Allah’s Apostle 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 Apostle 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.”
Hadith – Muslim & Bukhari
Abu Sa’id al-Khudri r.a. reported that the Prophet said: Whenever a Muslim is afflicted with a hardship, sickness, sadness, worry, harm, or depression –even a thorn’s prick, Allah expiates his sins because of it.
Recorded by Bukhari in al-Adab ul-Mufrad (no. 493), verified authentic by al-Albani
“Expect good, because Allah (s.w.t.) makes a believer’s sickness an expiation (for his sins) and a period of repose. As for a disbeliever falling sick, he is like a camel whose owner ties it and then lets it loose –it does not understand why it was tied, nor why it was freed.”