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 to him on his blessed hand for migration. 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." The Prophet (peace and blessings be upon him) refused. This happened three times. The fourth time, he did not come. When the Prophet (peace and blessings be upon him) inquired, the companions informed him that he had fled from 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 pure."