Musnad Ahmad 30:114

'Abd al-Rahman narrated to us, Malik narrated to us, from Yahya ibn Sa'id, from Sa'id ibn Yasar, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "I have been commanded to a town that eats the towns. They say it is Yathrib, and it is al-Madinah. It expels the people just as the bellows expels the dross of iron."