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.
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