Sahih Bukhari
Sahih Bukhari Volume 003, Book 030, Hadith Number 095.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 30. Virtues Of Madina
Narated By 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.