Musnad Ahmad 907:148

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 Prophet, peace and blessings of Allah be upon him, delivering the sermon standing upon the pulpit, then he would sit, then he would stand and deliver the sermon standing." He said: Jabir said to me: Whoever informs you that he used to deliver the sermon sitting, has lied. For by Allah, I have prayed with him more than two thousand prayers.