Sahih Bukhari
Virtues of Madinah
Volume: 003 / Book Number: 030 / Hadith Number: 95
Narrated Abu Huraira:
Allah's Apostle said, "I was ordered to migrate to a town which will swallow (conquer) other towns and is called Yathrib and that is Medina, and it turns out (bad) persons as a furnace removes the impurities of iron.