Musnad Ahmad 1243:7

Muhammad ibn Ja‘far narrated to us, he said: Shu‘bah narrated to us, from Yahya ibn al-Husayn, he said: I heard my grandmother narrating that she heard the Prophet, peace and blessings of Allah be upon him, delivering a sermon during the Farewell Pilgrimage, saying: "If a slave is appointed over you who leads you by the Book of Allah the Almighty and Majestic, then listen to him and obey him."