Sahih Bukhari Volume 7, Book 70, Hadith Number 566.

Narrated By Ibn ‘Abbas : Allah’s Apostle entered upon sick man to pay him a visit, and said to him, “Don’t worry, Allah willing, (your sickness will be) an expiation for your sins.” The man said, “No, it is but a fever that is boiling within an old man and will send him to his grave.” On that, the Prophet said, “Then yes, it is so.”