Silsilat al-Ahadith al-Sahihah

It is narrated from Sayyiduna Abdur Rahman bin Azhar (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "The example of a believing servant, when he is afflicted with fever, is like that of iron which is thrown into the fire; its impurities are removed and the useful part remains."