Virtues of Madinah

Bukhari :: Book 3 :: Volume 30 :: Hadith 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.