Musannaf Ibn Abi Shayba 27:785

Yahya ibn Sa'id narrated to us, from Muhammad ibn Yahya, from al-Harith ibn Abi Yazid, who heard Jabir ibn 'Abd Allah saying: The Messenger of Allah (may Allah bless him and grant him peace) said: "Al-Madinah is like the bellows; it expels the impurity, just as the bellows expels the impurity of iron."