Musnad Ahmad 907:41

Husayn narrated to us, from Za'idah, from Simak, from Jabir ibn Samurah, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) used to deliver the sermon on Friday while standing. So whoever tells you that he sat, then call him a liar." He said: And Jabir said: "The Messenger of Allah (peace and blessings of Allah be upon him) used to deliver two sermons; he would deliver a sermon, then sit, then stand and deliver a sermon. And the sermon of the Messenger of Allah (peace and blessings of Allah be upon him) and his prayer were of moderate length."