Musnad Abu Ya'la al-Mawsili 159:126

Ahmad ibn Isa narrated to us, Abdullah ibn Wahb narrated to us, Amr informed me that Yahya ibn Said narrated to him that Said ibn Yasar narrated to him that he heard Abu Hurayrah saying: The Messenger of Allah (may Allah bless him and grant him peace) said: "I have been commanded to a town that eats the towns; it is Yathrib, and it is al-Madinah. It expels the people just as the bellows expels the dross of iron."