Musnad Ahmad 907:139

Abdullah narrated to us, Abu al-Qasim al-Zuhri narrated to me, my uncle narrated to us, Sharik narrated to us, from Simak ibn Harb, from Jabir ibn Samurah, who said: Whoever tells you that he saw the Messenger of Allah, peace and blessings of Allah be upon him, delivering a sermon while sitting at all, then do not believe him. I have seen him more than a hundred times, and I saw him delivering the sermon while standing, then he would sit and not speak anything, then he would stand and deliver his other sermon. I said: How was his sermon? He said: It was moderate, speech with which he would admonish the people, and he would recite verses from the Book of Allah the Exalted.