It is narrated from Sayyiduna Jabir (may Allah be pleased with him) that a Bedouin came to the service of the Prophet (peace and blessings be upon him) and pledged allegiance on his blessed hand. After a short while, he developed a very high fever. He came to the Prophet (peace and blessings be upon him) and said, "Please annul my pledge of allegiance." The Prophet (peace and blessings be upon him) refused. This happened three times. The fourth time, he did not come. The Prophet (peace and blessings be upon him) inquired, and the Companions (may Allah be pleased with them) informed him that he had left Madinah. Upon this, the Prophet (peace and blessings be upon him) said, "Madinah is like a furnace; it removes its impurities and makes the good things shine and become pure."