It is narrated from Sayyiduna Jabir bin Abdullah (may Allah be pleased with him) that a Bedouin pledged allegiance to the Messenger of Allah (peace and blessings be upon him) upon Islam. He began to suffer from fever in Madinah. He came to the Prophet (peace and blessings be upon him) and said: O Messenger of Allah (peace and blessings be upon him)! Release me from my pledge. The Prophet (peace and blessings be upon him) refused. He came again and said: Release me from my pledge. The Prophet (peace and blessings be upon him) refused. He came again and said: Release me from my pledge. The Prophet (peace and blessings be upon him) refused. He then left Madinah. At that time, the Prophet (peace and blessings be upon him) said: Madinah is like a bellows or a furnace; it removes impurities and retains only the pure.