Musnad Ahmad 28:339

Waki' 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, who said: The Messenger of Allah (ﷺ) said: "Whoever pledges allegiance to an Imam, giving him the fruit of his heart and the clasp of his hand, then let him obey him as much as he is able."