Musnad Ahmad 33:820

Abu Nu'aym narrated to us, Sufyan narrated to us, from Muhammad ibn al-Munkadir, he said: I heard Jabir say: A Bedouin came to the Messenger of Allah, may Allah bless him and grant him peace, and said: "Pledge allegiance to me upon Islam." So he pledged allegiance to him upon Islam. Then he came the next day with a fever, and said: "O Messenger of Allah, release me [from my pledge]." But he refused. Then he came the next day with a fever, and said: "Release me." But he refused. When he turned away, he [the Prophet] said: "Al-Madinah is like the bellows; it expels its filth and purifies its good."