Musnad Ahmad 907:28

'Affan narrated to us, Abu 'Awanah narrated to us, Simak bin Harb narrated to us, from Jabir bin Samurah, who said: "I saw the Messenger of Allah, may Allah bless him and grant him peace, delivering the sermon standing. Then he would sit for a sitting in which he would not speak. Then he would stand and deliver another sermon upon his pulpit. So whoever tells you that he saw him delivering the sermon sitting, do not believe him."