Muwatta Imam Malik (Riwayat Ibn al-Qasim)

It is narrated from Sayyiduna Jabir bin Abdullah (al-Ansari, may Allah be pleased with him) that a Bedouin pledged allegiance to the Messenger of Allah, peace and blessings be upon him, for Islam (and to remain in Madinah). Then that Bedouin became feverish in Madinah, so he came to the Prophet, peace and blessings be upon him, and said: "O Messenger of Allah! Cancel my pledge." The Prophet, peace and blessings be upon him, refused, so he came again and said: "Cancel my pledge." The Prophet, peace and blessings be upon him, refused, so he came a third time and said: "Cancel my pledge." The Prophet, peace and blessings be upon him, refused, then that Bedouin left (Madinah) and went away. The Prophet, peace and blessings be upon him, then said: "Madinah is like a furnace of a goldsmith; it removes impurities and filth and refines and brightens the good."