Allâh makes the illness of a believer an expiation…
Salmân Al-Fârsî – Allâh be pleased with him – once visited a sick friend. When he entered upon him he said:
Have glad tidings, for verily Allâh makes the illness of a believer an expiation [for his sins] and a cause of being pleased, whereas the illness of a sinner is like a camel that has been tied by its owners, thenreleased by them: it knows not why it was tied up nor why it was released.
Al-Bukhârî, Al-Adab Al-Mufrad in the Chapter on the Expiation [of sins through] illness