It is narrated from Sayyiduna Jabir bin Abdullah (may Allah be pleased with him) that a Bedouin came to the service of the Noble Prophet (peace and blessings be upon him) and pledged allegiance to Islam. Later, he developed a fever. He came to the Noble Prophet (peace and blessings be upon him) and said: "Release me from my pledge." The Prophet (peace and blessings be upon him) refused. He came again, and the Prophet (peace and blessings be upon him) again refused. He said: "Release me from my pledge," and the Prophet (peace and blessings be upon him) refused. Then the Prophet (peace and blessings be upon him) inquired about him, and the companions informed him that he had left. Upon this, the Prophet (peace and blessings be upon him) said: "Medina is like a furnace; it expels bad people and reveals good people."