Musnad Ahmad 33:188

Sufyan narrated to us, Ibn al-Munkadir narrated to us, he said: I heard Jabir saying: A man from the Bedouins came to the Messenger of Allah, may Allah bless him and grant him peace, and accepted Islam. He pledged allegiance to him upon migration. He did not remain long before he caught a fever, so he came to the Prophet, may Allah bless him and grant him peace, and said: "Release me [from my pledge]." He said: "I will not release you." Then he came to him and said: "Release me." He said: "I will not release you." Then he came to him and said: "Release me." He said: "No." So he fled. Then he [the Prophet] said: "Medina is like the bellows; it expels its filth and purifies its good."