Musnad Ahmad 938:61

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from ‘Adi ibn Thabit, from ‘Abdullah ibn Yazid, narrating from Zayd ibn Thabit, that he said regarding this verse: {Why are you then two parties regarding the hypocrites, while Allah has cast them back [to disbelief] for what they earned?} [An-Nisa: 88]. He said: Some people from the companions of the Prophet, peace and blessings of Allah be upon him, returned [from the Battle of Uhud], and the people were divided into two groups regarding them. One group said: "Kill them," and one group said: "No." He said: So this verse was revealed: {Why are you then two parties regarding the hypocrites}. And he said: "It [Medina] is Tayyibah (the good one), and it expels the filth, just as the fire expels the filth of silver."