Musnad Abu Ya'la al-Mawsili 152:258

Abu Khaythama narrated to us, Wahb ibn Jarir narrated to us, my father narrated to us, he said: I heard Abd al-Malik ibn Umayr narrating from Abd al-Rahman ibn Abd Allah, from his father, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Beware, do not return after me as disbelievers, striking the necks of one another."