From Sahih Bukhari (Hadith 7470):
Narrated Ibn `Abbas:Allah's Messenger (ﷺ) 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 (ﷺ) said, "Then it is so
Reference: Bukhari 7470