Sahih Bukhari Volume 7, Book 70, Hadith Number 560.

Narrated By Ibn Abbas : The Prophet went to visit a sick bedouin. Whenever the Prophet went to a patient, he used to say to him, “Don’t worry, if Allah will, it will be expiation (for your sins):” The bedouin said, “You say expiation? No, it is but a fever that is boiling or harassing an old man and will lead him to his grave without his will.” The Prophet said, “Then, yes, it is so.”