I used to stand in attendance over Mukhtar. When his lying became clear to me, I resolved, by Allah, to draw my sword and strike off his neck, until I recalled a hadith that Amr bin Hamiq narrated to me. He said, "I heard the Messenger of Allah ﷺ say, 'Whoever grants a man security for his life and then kills him will be given the banner of treachery on the Day of Resurrection.'"
حدثنا بهز بن أسد حدثنا حماد بن سلمة عن عبد الملك بن عمير
كنت أقوم على رأس المختار فلما تبينت كذابته هممت وايم الله أن أسل سيفي فأضرب عنقه حتى ذكرت حديثا حدثنيه عمرو بن الحمق قال سمعت رسول الله ﷺ يقول من أمن رجلا على نفسه فقتله أعطي لواء الغدر يوم القيامة
Its isnad is sahih, and it is found in Nasai in Al-Kubra, 5/225, no. 8739, and Ibn Majah, 2/896, no. 2688, where he said in Al-Zawaid, "Its narrators are trustworthy. Rifa bin Shaddad was cited by Nasai, who deemed him trustworthy, and Ibn Hibban mentioned him in Al-Thiqat."