From Sahih Bukhari (Hadith 7211):
Narrated Jabir bin `Abdullah:A bedouin gave the Pledge of allegiance to Allah's Messenger (ﷺ) for Islam. Then the bedouin got fever at Medina, came to Allah's Messenger (ﷺ) and said, "O Allah's Messenger (ﷺ)! Cancel my Pledge," But Allah's Apostle refused. Then he came to him (again) and said, "O Allah's Messenger (ﷺ)! Cancel my Pledge." But the Prophet (ﷺ) refused Then he came to him (again) and said, "O Allah's Messenger (ﷺ)! Cancel my Pledge." But the Prophet (ﷺ) refused. The bedouin finally went out (of Medina) whereupon Allah's Messenger (ﷺ) said, "Medina is like a pair of bellows (furnace): It expels its impurities and brightens and clears its good
Reference: Bukhari 7211