Musnad Ahmad 1243:2

Abu Qatan narrated to us, he said: Yunus—meaning Ibn Abi Ishaq—narrated to us, from al-Aizar ibn Hurayth, from Umm al-Husayn al-Ahmasiyyah, who said: I saw the Messenger of Allah (peace and blessings of Allah be upon him) during the Farewell Pilgrimage, delivering a sermon upon the pulpit, wearing a cloak of his that he had wrapped around himself from under his armpit. She said: I was looking at the muscle of his upper arm trembling, and I heard him say: "O people, fear Allah, and if a mutilated slave is appointed in authority over you, then listen to him and obey him, as long as he upholds the Book of Allah, the Almighty and Majestic, among you."