Sahih Bukhari | Volume 3 | Hadith Number 108

Narrated by 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."