Abu Burdah bin Abi Musa (may Allah have mercy on him) said: I went in to Mu'awiyah bin Abi Sufyan (may Allah be pleased with him). There was a sore on his back and he was groaning severely because of it. I said: Is all this because of this pain? He said: I would not be pleased had this groaning not happened. Then he said: I heard the Messenger of Allah (peace and blessings be upon him) say: "No Muslim is afflicted with any harm in his body except that it becomes an expiation for his sins," and this is the most severe harm.
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 16891