Musnad Ahmad 27:1359

'Affan narrated to us, Shu'bah narrated to us, from Waqid ibn 'Abdullah—thus said 'Affan, but it is actually Waqid ibn Muhammad ibn Zayd ibn 'Abdullah ibn 'Umar—from his father, that he heard 'Abdullah ibn 'Umar, from the Prophet (peace and blessings of Allah be upon him), who said: "Do not return after me as disbelievers, striking the necks of one another."