Abu Huraira (Allah be pleased with him) reported Allah's Messenger (way peace be upon him) as saying:I have been commanded (to migrate) to a town (Medina) which would overpower other towns. They (the people) call it Yathrib; its correct name is (in fact) Medina. It eliminates (bad) people just as a furnace removes the alloy of iron

This hadith has been narrated by Yabya b. Sa'id with the same chain of transmitters (and the words are):" Just as a furance removes impurity," but no mention is made of iron