Musnad Ahmad 33:172

'Abd al-Rahman narrated to us, Malik narrated to us, from Muhammad ibn al-Munkadir, who said: I heard Jabir ibn 'Abd Allah saying: A Bedouin came to the Prophet, peace and blessings of Allah be upon him, and pledged allegiance to him upon Islam. Then he was struck by fever, so he came to the Prophet, peace and blessings of Allah be upon him, and said: "Cancel my pledge." But he refused. Then he came to him again and said: "Cancel my pledge." But he refused. Then he asked about him, and they said: "He has left." So the Messenger of Allah, peace and blessings of Allah be upon him, said: "Verily, Medina is like a bellows; it expels its impurity and refines its purity."