Musnad Ahmad 1243:4

Waki’ narrated to us, from Isra’il, from Abu Ishaq, from Yahya ibn al-Husayn, from his mother, who said: I heard the Prophet, peace and blessings of Allah be upon him, at Arafat, delivering a sermon during the Farewell Pilgrimage, saying: "O people, fear Allah, and listen and obey, even if an Abyssinian slave whose nose and ears have been cut off is appointed over you, as long as he establishes the Book of Allah, the Almighty and Majestic, among you."