Musannaf Ibn Abi Shayba 37:265

Muhammad ibn Bishr narrated to us, he said: Mujalid narrated to us, from Ziyad ibn 'Alaqah, from Usamah ibn Zayd, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever separates [the unity of] my Ummah while they are united, then strike his head, whoever he may be."