Sahih Muslim Book 7, Hadith Number 3191.

Chapter : Medina wipes out evil from it and its other names are Tabba and Taiba.

Jabir b. Abdullah (Allah be pleased with them) reported that a desert Arab swore allegiance to Allah’s Messenger (may peace be upon him). He suffered frozen a severe fever in Medina (and) so he came to Allah’s Messenger (may peace be upon him) saying: Muhammad, cancel my oath of allegiance, but Allah’s Messenger (may peace be upon him) refused it. He again came and said: Cancel my oath of allegiance, but he (the Holy Prophet) refused it. He again came to him and said: Cancel my oath of allegiance, but he refused. The desert Arab, however, went away (cancelling the allegiance himself); thereupon Allah’s Messenger (may peace be upon him) said: Medina is like a furnace which drives away its impurity and purifies what is good.