(10) باب الْمَدِينَةُ تَنْفِي الْخَبَثَ
Narrated Jabir:
A bedouin came to the Prophet (ﷺ) and gave a pledge of allegiance for embracing Islam. The next day he came with fever and said (to the Prophet (ﷺ) ), "Please cancel my pledge (of embracing Islam and of emigrating to Medina)." The Prophet (ﷺ) refused (that request) three times and said, "Medina is like a furnace, it expels out the impurities (bad persons) and selects the good ones and makes them perfect."
Reference: Sahih al-Bukhari 1883
In-book reference: Book 29, Hadith 17
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 3, Book 30, Hadith 107