The two illness

Salman al Farisi ( Radia Allahu anhu) once visited a sick friend. When he entered upon him he said:

"Have glad tidings , for verily Allah makes the illnes of the 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 , then released by them: It knows not why it was tied up nor why it was released."

SOURCE:  Al-Bukhârî, Al-Adab Al-Mufrad in the Chapter on the Expiation [of sins through] illness. Graded sahîh by Shaykh Al-Albânî in Sahîh Al-Adab Al-Mufrad.