Sahih Bukhari Volume 6, Book 65, Hadith 4591

حَدَّثَنِي مُحَمَّدُ بْنُ بَشَّارٍ، حَدَّثَنَا غُنْدَرٌ، وَعَبْدُ الرَّحْمَنِ، قَالاَ حَدَّثَنَا شُعْبَةُ، عَنْ عَدِيٍّ، عَنْ عَبْدِ اللَّهِ بْنِ يَزِيدَ، عَنْ زَيْدِ بْنِ ثَابِت ٍ ـ رضى الله عنه ـ ‏{‏فَمَا لَكُمْ فِي الْمُنَافِقِينَ فِئَتَيْنِ‏}‏ رَجَعَ نَاسٌ مِنْ أَصْحَابِ النَّبِيِّ صلى الله عليه وسلم 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 ‏"‏‏.‏

Narrated By Zaid bin Thabit : Regarding the Verse: "Then what is the matter with you that you are divided into two parties about the hypocrites?" (4.88) Some of the companions of the Prophet returned from the battle of Uhud (i.e. refused to fight) whereupon the Muslims got divided into two parties; one of them was in favour of their execution and the other was not in favour of it. So there was revealed: "Then what is the matter with you that you are divided into two parties about the hypocrites?" (4.88). Then the Prophet said "It (i.e. Medina) is a Tayyaboh (good), it expels impurities as the fire expels the impurities of silver."