It is narrated from Sayyiduna Jabir (may Allah be pleased with him) that some people came to the service of the Prophet (peace and blessings be upon him) and pledged allegiance at his blessed hand. At that time, there was an epidemic in Madinah, so the Prophet (peace and blessings be upon him) forbade them from leaving Madinah Munawwarah without permission. However, they left Madinah without taking permission. Upon this, the Prophet (peace and blessings be upon him) said that Madinah Munawwarah is like a furnace which removes its impurities.