Abdullah bin Wahb عبد الله بن وهب بن مسلم القرشي · The Book of al-Bay'ah

"We pledged to the Messenger of Allah to hear and obey, both in times of hardship and ease." And he mentioned similarly.

"I heard my grandmother say: 'I heard the Messenger of Allah say, during the Farewell Pilgrimage: If an Ethiopian slave is appointed over you who rules according to the Book of Allah, then listen to him and obey."'