Musnad Ahmad 399:1

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