Volume 9, Book 89, Number 316:
Narrated 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.”