Mishkat al-Masabih #1491

‘Amir ar-Ram told that God’s messenger mentioned illnesses and said, "When a believer is afflicted by illness and God who is great and glorious cures him of it, it serves as an atonement for his previous sins and a warning to him for the future; but when a hypocrite becomes ill and then is cured he is like a camel which has been tethered and then let loose by its owners, but does not know why they tethered it and why they let it loose.” When a man asked God’s messenger what illnesses were, adding that he swore by God he had never been ill, he said, "Get up and leave us ; you do not belong to our number.” Abu Dawud transmitted it.