Sahih Bukhari
Virtues of Madinah
Volume: 003 / Book Number: 030 / Hadith Number: 108
Narrated Zaid bin Thabit:
When the Prophet went out for (the battle of) Uhud, some of his companions (hypocrites) returned (home). A party of the believers remarked that they would kill those (hypocrites) who had returned, but another party said that they would not kill them. So, this Divine Inspiration was revealed: "Then what is the matter with you that you are divided into two parties concerning the hypocrites." (4.88) The Prophet said, "Medina expels the bad persons from it, as fire expels the impurities of iron."