Musnad Ahmad 26:95

Abu Mu'awiyah narrated to us, Al-A'mash narrated to us, from Abu Ishaq, from Harithah ibn Mudharrib, who said: Abdullah said to Ibn al-Nawwahah: "I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'Were it not that you are a messenger [envoy], I would have killed you.' As for today, you are not a messenger." "O Kharashah, stand up and strike his neck." He said: So he stood up to him and struck his neck.