Musnad Ahmad 938:24

Bahz narrated to us, Shu'bah narrated to us, 'Adi ibn Thabit said: He informed me from 'Abdullah ibn Yazid, from Zayd ibn Thabit: That the Messenger of Allah (peace be upon him) went out to Uhud, and some people who had gone out with him returned. The companions of the Messenger of Allah (peace be upon him) were divided into two groups regarding them: a group saying to kill them, and a group saying: No. Then Allah, the Almighty and Majestic, revealed: {So what is the matter with you [that you are] two groups concerning 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."