Musnad Ahmad 30:252

Sufyan narrated to us, from Yahya ibn Sa'id, from Sa'id ibn Yasar, from Abu Hurayrah, from the Prophet (peace and blessings of Allah be upon him): "I have been commanded [to go to] a village that eats the villages. They say it is Yathrib, and it is Al-Madinah. It expels the people just as the bellows expels the dross of iron."