Musnad Ahmad 938:59

'Affan narrated to us, Shu'bah narrated to us, he said: 'Adi ibn Thabit informed me, he said: I heard 'Abdullah ibn Yazid, from Zayd ibn Thabit, who said: When the Messenger of Allah (peace be upon him) went out to Uhud, some people who had gone out with him returned. The companions of the Messenger of Allah (peace be upon him) became two factions: a faction saying, "We shall kill them," and a faction saying, "No." Ibn Ja'far said: A group said, "Kill them," and a group said, "No." Bahz said: Then Allah revealed: "Then what is the matter with you [that you are] two factions regarding the hypocrites?" [An-Nisa: 88]. The Messenger of Allah (peace be upon him) said: "It is Tayyibah (Madinah), and it expels the filth, just as the fire expels the filth of silver."