From Sahih Bukhari (Hadith 7322):
Narrated Jabir bin 'Abdullah As-Salami: A bedouin gave the Pledge of allegiance for embracing Islam to Allah's Messenger (ﷺ), and then he got an attack of fever in Medina and came to Allah's Messenger (ﷺ): and said, "O Allah's Messenger (ﷺ)! Cancel my pledge." Allah's Messenger (ﷺ) refused to do so. The bedouin came to him again and said, "Cancel my pledge," but he refused again, and then again, the bedouin came to him and said, "Cancel my pledge," and Allah's Messenger (ﷺ) refused. The bedouin finally went away, and Allah's Messenger (ﷺ) said, "Medina is like a pair of bellows (furnace), it expels its impurities while it brightens and clears its good
Reference: Bukhari 7322