Some of the companions of the Prophet ﷺ returned from the battle of Uhud (i.e. refused to fight) whereupon the Muslims got divided into two parties; one of them was in favor of their execution and the other was not in favour of it. So there ware revealed: "Then what is the matter with you that you are divided into two parties about the hypocrites?" (4.88). Then the Prophet ﷺ said "It (i.e. Medina) is a Taybah (good), it expels impurities as the fire expels the impurities of silver."
رَجَعَ نَاسٌ مِنْ أَصْحَابِ النَّبِيِّ ﷺ مِنْ أُحُدٍ وَكَانَ النَّاسُ فِيهِمْ فِرْقَتَيْنِ فَرِيقٌ يَقُولُ اقْتُلْهُمْ وَفَرِيقٌ يَقُولُ لاَ۔ فَنَزَلَتْ {فَمَا لَكُمْ فِي الْمُنَافِقِينَ فِئَتَيْنِ} وَقَالَ: «إِنَّهَا طَيْبَةُ: تَنْفِي الْخَبَثَ كَمَا تَنْفِي النَّارُ خَبَثَ الْفِضَّةِ۔»