Musnad Ahmad 28:25

Abu Mu'awiyah narrated to us, Al-A'mash narrated to us, from Zayd ibn Wahb, from 'Abd al-Rahman ibn 'Abd Rabb al-Ka'bah, from 'Abdullah ibn 'Amr ibn al-'As, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever pledges allegiance to an Imam, giving him the clasp of his hand and the fruit of his heart, then let him obey him as much as he is able. If another comes to dispute with him, then strike the neck of the other."