Musnad Ahmad 907:60

Abu Sa’id narrated to us, Za’idah narrated to us, Simak narrated to us, from Jabir ibn Samurah, who said: "I never saw the Messenger of Allah, may Allah bless him and grant him peace, ever deliver a sermon on Friday except while standing. So whoever tells you that he sat down, then call him a liar, for he did not do that. The Prophet, may Allah bless him and grant him peace, would deliver a sermon, then sit, then stand and deliver a sermon. He would deliver two sermons, sitting between them on Friday."