Musnad Ahmad 907:37

Abu Kamil narrated to us, Zuhayr narrated to us, Simak ibn Harb narrated to us, he said: Jabir ibn Samurah informed me that he saw the Messenger of Allah (may Allah bless him and grant him peace) deliver a sermon standing on the pulpit, then he would sit, then he would stand, and deliver a sermon standing. He said: Jabir said to me: "Whoever informs you that he used to deliver a sermon while sitting has lied. For by Allah, I have prayed with him more than two thousand prayers."