Yahya related to me from Malik from Ibn Shihab from Ata ibn Yazidal-Laythi that Ubaydullah ibn Adi ibnal-Khiyar said, "Once when theMessenger of Allah, may Allah bless him and grant him peace, wassitting with some people, a man came to him and spoke secretly to him.Nobody knew what he had said until the Messenger of Allah, may Allahbless him and grant him peace, disclosed that he had asked forpermission to kill one of the hypocrites. When he disclosed this, theMessenger of Allah, may Allah bless him and grant him peace, said,'Doesn't he testify that there is no god but Allah and that Muhammadis the Messenger of Allah?' The man replied, 'Of course, but hehasn't really done so.' He said, 'Doesn't he do the prayer?' and theman replied, 'Of course, but he doesn't really do the prayer.' Hesaid, may Allah bless him and grant him peace, 'Those are the oneswhom Allah has forbidden me (to kill).' "
USC-MSA web (English) reference: Book 9, Hadith 87
Arabic reference: Book 9, Hadith 418