Sahih Bukhari Volume 009, Book 089, Hadith Number 323.
Narrated By Jabir : A bedouin came to the Prophet and said, "Please take my Pledge of allegiance for Islam." So the Prophet took from him the Pledge of allegiance for Islam. He came the next day with a fever and said to the Prophet "Cancel my pledge." But the Prophet refused and when the bedouin went away, the Prophet said, "Medina is like a pair of bellows (furnace): It expels its impurities and brightens and clears its good."