Musnad Ahmad 1243:8

Waki’ narrated to us, from Yunus, from al-Aizar ibn Hurayth, from Umm al-Husayn al-Ahmasiyyah, who said: I heard the Prophet, peace and blessings of Allah be upon him, while he was standing at Arafat, wearing a mantle (burdah) which he had wrapped around himself, and he was saying: "Listen and obey, even if an Abyssinian slave is appointed over you, as long as he establishes the Book of Allah among you."