Musannaf Ibn Abi Shayba 28:8

Waki’ and Abu Mu’awiyah narrated to us, from al-A’mash, from Zayd ibn Wahb, from ‘Abd al-Rahman ibn ‘Abd Rabb al-Ka’bah, from ‘Abdullah ibn ‘Amr, who said: The Messenger of Allah (may Allah bless him and grant him peace) 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."