Sahih al-Bukhari 7216

Narrated Jabir:

A bedouin came to the Prophet (ﷺ) 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 (ﷺ) “Cancel my pledge.” But the Prophet (ﷺ) 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.”