Sahih Muslim Book 7, Hadith Number 3189.
Chapter : Medina wipes out evil from it and its other names are Tabba and Taiba.
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.