Musannaf Ibn Abi Shayba 28:9

Waki' narrated to us, he said: Shu'bah narrated to us, from Yahya ibn al-Husayn, from his grandmother, Umm al-Husayn, she said: I heard the Prophet, may Allah bless him and grant him peace, while he was delivering a sermon at Arafat, and he was saying: "If an Abyssinian slave is appointed in command over you, then listen to him and obey him, as long as he leads you by the Book of Allah."