Musnad Ahmad 907:113

Abdullah narrated to us, Khalaf ibn Hisham 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 the sermon standing. He would sit for a sitting [period] in which he would not speak, then he would stand and deliver another sermon standing. So whoever tells you that the Messenger of Allah, may Allah bless him and grant him peace, delivered the sermon sitting, do not believe him."