Musannaf Ibn Abi Shayba 36:255

Abu Usama narrated to us, from Shu'bah, from 'Adi ibn Thabit, from 'Abdullah ibn Yazid, from Zayd ibn Thabit, who said: When the Messenger of Allah (peace be upon him) went out to Uhud, some people went out with him, then they returned. The companions of the Messenger of Allah (peace be upon him) were divided into two groups regarding them. One group said, "We will kill them," and one group said, "We will not kill them." Then it was revealed: "So what is the matter with you [that you are] two groups concerning the hypocrites, while Allah has made them fall back [into error and disbelief] for what they earned?" He said: The Messenger of Allah (peace be upon him) then said, "Indeed, it is Tayyibah (Madinah), and it expels the filth just as the fire expels the filth of silver."