From Sahih Bukhari (Hadith 1871):
Narrated Abu Huraira:Allah's Messenger (ﷺ) 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
Reference: Bukhari 1871