It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) went to visit a Bedouin who was sick. The narrator said: Whenever he (peace and blessings be upon him) visited a sick person, he would say: "No harm; if Allah wills, it (this illness) is a purification (from sins)." The man said: You say purification? No, rather it is a fever boiling within an old man, which will make him visit the graves. The Prophet (peace and blessings be upon him) said: "Then so it shall be."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11901