Musnad Ahmad 1243:12

Waki’ narrated to us, Shu’bah said: I came to Yahya ibn al-Husayn and asked him, so he said: My grandmother narrated to me, she said: I heard the Prophet, may Allah bless him and grant him peace, saying while he was standing at Arafat: "If an Abyssinian slave is appointed in authority over you, then listen to him and obey him, as long as he leads you by the Book of Allah the Exalted."