Musnad Abu Ya'la al-Mawsili 248:1

Muhammad ibn Ubayd ibn Hisab narrated to us, Abu 'Awanah narrated to us, from Simak, from Jabir ibn Samurah, who said: I saw the Messenger of Allah (may Allah bless him and grant him peace) delivering a sermon while standing, then he would sit and not speak, then he would stand and deliver another sermon upon his pulpit. So whoever tells you that he saw him delivering a sermon while sitting, do not believe him.