Sayyiduna Abd al-Rahman bin Azhar (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) said: "When a believer is afflicted with a severe fever or a fever, the example of (that sick believer) is like iron that goes into the fire: its impurity (that is, its rust) goes away and what is pure remains."
Al-Bazzar and al-Tabarani recorded it in al-Mu'jam al-Kabir. Its chain contains a narrator who is not known.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الجنائز / 3801