Narrated 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 (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 aTayyaboh (good), it expels impurities as the fire expels the impurities of silver."
USC-MSA web (English) reference: Vol. 6, Book 60, Hadith 113
Arabic reference: Book 65, Hadith 4589