Musnad Ahmad 907:17

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Simak ibn Harb, who said: I asked Jabir ibn Samurah, "How did the Messenger of Allah, peace and blessings of Allah be upon him, deliver the sermon?" He said: "He used to deliver the sermon standing, except that he would sit for a sitting, then he would stand."