Musnad Abu Ya'la al-Mawsili 153:178

Ahmad ibn Ibrahim al-Dawraqi narrated to us, Abu Dawud narrated to us, Asim ibn Muhammad narrated to us, from his father, from Ibn Umar, who said: I heard the Messenger of Allah (peace and blessings of Allah be upon him) saying: "Do not return after me as disbelievers, striking the necks of one another."