Sahih Bukhari Volume 9, Book 93, Number 562:


Narrated Ibn 'Abbas:

Allah's Apostle entered upon a sick bedouin in whom he went to visit and said to him, "Don't worry, Tahur (i.e., your illness will be a means of cleansing of your sins), if Allah Will." The bedouin said, "Tahur! No, but it is a fever that is burning in the body of an old man and it will make him visit his grave." The Prophet said, "Then it is so."