Hadith 160
“the Messenger of Allah (ﷺ) went to visit a Bedouin when he was ill and said, "Do not worry. It is a purification if Allah wills." the Bedouin said, "It is a fever which boils in an old man and will cause him to visit the graves." he (ﷺ) said: “yes, it is a blessing then,”
Ibn ‘Abbas (رضي الله عنه)Al-Adab Al-Mufrad 514Adab & Virtues on Sickness