Musannaf Ibn Abi Shayba 28:10

Waki' narrated to us, he said: Yunus bin Abi Ishaq narrated to us, from al-Aizar bin Hurayth al-'Abdi, from Umm al-Husayn al-Ahmasiyyah, she said: I heard the Prophet, may Allah bless him and grant him peace, while he was delivering the sermon at Arafat, and he was wrapped in a garment, saying: "If an Abyssinian slave, who is mutilated, is appointed in command over you, then listen to him and obey him, as long as he leads you by the Book of Allah."