Sahih Muslim Book 7, Hadith Number 3188.

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 (may peace be upon him) as saying: A time will come for the people (of Medina) when a man will invite his cousin and any other near relation: Come (and settle) at (a place) where living is cheap, come to where there is plenty, but Medina will be better for them; would they know it! By Him in Whose Hand is my life, none amongst them would go out (of the city) with a dislike for it, but Allah would make his successor in it someone better than be. Behold. Medina is like furnace which eliminates from it the impurities. And the Last Hour will not come until Medina banishes its evils just as a furnace eliminates the impurities of iron.