Sahih Bukhari Volume 009, Book 089, Hadith Number 316.
Narrated By Jabir bin 'Abdullah : A bedouin gave the Pledge of allegiance to Allah's Apostle for Islam and the bedouin got a fever where upon he said to the Prophet "Cancel my Pledge." But the Prophet refused. He came to him (again) saying, "Cancel my Pledge.' But the Prophet refused. Then (the bedouin) left (Medina). Allah's Apostle said: "Medina is like a pair of bellows (furnace): It expels its impurities and brightens and clears its good."