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."
Sahih al-Bukhari Book 29, Hadith 17
← Previous Hadith Allah's Messenger (ﷺ) told us a long narrative about Ad-Dajjal, and among the many things he mentioned, was his saying, "Ad-Dajjal will come and it will be forbidden for him to pass through the entrances of Medina. He will land in some of the salty barren areas (outside) Medina; on that day the best man or one of the best men will come up to him and say, 'I testify that you are the same Dajjal whose description was given to us by Allah's Messenger (ﷺ) .' Ad-Dajjal will say to the people, 'If I kill this man and bring him back to life again, will you doubt my claim?' They will say, 'No.' Then Ad-Dajjal will kill that man and bring him back to life. That man will say, 'Now I know your reality better than before.' Ad-Dajjal will say, 'I want to kill him but I cannot.' "

Sahih al-Bukhari Book 29, Hadith 16 Next Hadith → When the Prophet (ﷺ) went out for (the battle of) Uhud, some of his companions (hypocrites) returned (home). A party of the believers remarked that they would kill those (hypocrites) who had returned, but another party said that they would not kill them. So, this Divine Inspiration was revealed: "Then what is the matter with you that you are divided into two parties concerning the hypocrites." (4.88) The Prophet (ﷺ) said, "Medina expels the bad persons from it, as fire expels the impurities of iron."

Sahih al-Bukhari Book 29, Hadith 18