Musannaf Ibn Abi Shayba 3:2803

Ali ibn Mushir narrated to us, from Layth, from Tawus, who said: The Messenger of Allah (peace and blessings of Allah be upon him) delivered the sermon while standing, and Abu Bakr delivered it while standing, and Umar delivered it while standing. The first person to sit on the pulpit was Muawiyah ibn Abi Sufyan.