Musnad Ahmad 27:1360

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from Waqid ibn Muhammad ibn Zayd, that he heard his father narrating from ‘Abdullah ibn ‘Umar, from the Prophet, may Allah bless him and grant him peace, that he said during the Farewell Pilgrimage: "Woe to you — or he said: Woe unto you — do not return after me as disbelievers, striking the necks of one another."