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."  

عَنْ جَابِرٍ ؓ جَاءَ أَعْرَابِيٌّ النَّبِيَّ ﷺ فَبَايَعَهُ عَلَى الإِسْلاَمِ فَجَاءَ مِنَ الْغَدِ مَحْمُومًا فَقَالَ أَقِلْنِي فَأَبَى ثَلاَثَ مِرَارٍ فَقَالَ الْمَدِينَةُ كَالْكِيرِ تَنْفِي خَبَثَهَا وَيَنْصَعُ طَيِّبُهَا  

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."  

سَمِعْتُ زَيْدَ بْنَ ثَابِتٍ ؓ يَقُولُ لَمَّا خَرَجَ النَّبِيُّ ﷺ إِلَى أُحُدٍ رَجَعَ نَاسٌ مِنْ أَصْحَابِهِ فَقَالَتْ فِرْقَةٌ نَقْتُلُهُمْ وَقَالَتْ فِرْقَةٌ لاَ نَقْتُلُهُمْ فَنَزَلَتْ {فَمَا لَكُمْ فِي الْمُنَافِقِينِ فِئَتَيْنِ} وَقَالَ النَّبِيُّ ﷺ إِنَّهَا تَنْفِي الرِّجَالَ كَمَا تَنْفِي النَّارُ خَبَثَ الْحَدِيدِ