Musnad Ahmad 33:1100

'Abd al-Razzaq narrated to us, Sufyan informed us, from Ibn al-Munkadir, from Jabir, who said: A Bedouin came to the Prophet, may Allah bless him and grant him peace, and pledged allegiance to him upon Islam. The next day, he came suffering from a fever and said: "O Messenger of Allah, release me [from my pledge]." He refused. He came to him for three consecutive days, and each time he said: "O Messenger of Allah, release me," and the Prophet, may Allah bless him and grant him peace, refused. When he turned away, the Prophet, may Allah bless him and grant him peace, said: "Verily, Medina is like a bellows; it expels its filth and purifies its good."