Abdur Rahman ibn Azhar رحمه الله تعالى reported: Rasulullah (صلى الله عليه وسلم) said,
“Verily the parable of the believer, when he is afflicted by illness or fever, is only that of a piece of iron placed in the fire; its defects vanish and its goodness remains.”
(As Sunanul Kubra, vol. 3 pg. 374)