Narrated from Umm al-Mu'minin Aisha Siddiqa (may Allah be pleased with her): I have not seen the severity of illness upon anyone as much as I saw upon the Messenger of Allah (peace and blessings be upon him).

Narrated Abdullah (bin Mas'ud) (may Allah be pleased with him):
I visited the Prophet (peace and blessings of Allah be upon him) during his illness, and he had a very high fever. I said, "You have a very severe fever, O Messenger of Allah." (Perhaps) it is because you will have a double reward. He said: "Yes! No affliction befalls a Muslim but that Allah expiates his sins because of it, just as the leaves of a dry tree fall off."