Sayyiduna Abd al-Rahman bin Azhar (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "The example of the believer is like that of iron: when fever or illness strikes him, his impurity (his faults) goes away and his purity remains." That is, just as when iron is put into fire its dross goes away and the pure iron is left.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنائز / 6544