Musnad Ahmad 33:1015

Husayn ibn Muhammad narrated to us, al-Fudayl—meaning Ibn Sulayman—narrated to us, Muhammad ibn Abi Yahya narrated to us, from al-Harith ibn Abi Yazid, from Jabir ibn Abdullah al-Ansari, that a group of people came to Medina with the Prophet, may Allah bless him and grant him peace, and there was sickness in it. The Prophet, may Allah bless him and grant him peace, forbade them from leaving until he gave them permission, but they left without his permission. The Messenger of Allah, may Allah bless him and grant him peace, said: "Medina is only like the bellows; it expels the impurity, just as the bellows expels the impurity of iron."