Musnad Ahmad 1243:10

Abu Nu'aym narrated to us, he said: Yunus narrated to us, from al-Ayzar ibn Hurayth, he said: I heard Umm al-Husayn al-Ahmasiyyah say: I saw the Messenger of Allah, peace and blessings of Allah be upon him, during the Farewell Pilgrimage, wearing a cloak that he had wrapped around himself from under his armpit. I was looking at the muscle of his upper arm trembling, while he was saying: "O people, fear Allah, and listen and obey, even if an Abyssinian slave with severed limbs is appointed over you; so listen and obey as long as he upholds the Book of Allah among you."