Musnad Ahmad 907:68

Husayn ibn Ali narrated to us, from Za'idah, from Simak, from Jabir ibn Samurah, who said: "I never saw the Messenger of Allah, may Allah bless him and grant him peace, deliver a sermon on Friday except while standing. So whoever tells you that he saw him delivering a sermon while sitting, he has lied." He said: And Simak said: Jabir ibn Samurah said: "The prayer of the Messenger of Allah, may Allah bless him and grant him peace, and his sermon were of moderate length." And Jabir ibn Samurah said: "The Messenger of Allah, may Allah bless him and grant him peace, used to deliver the sermon standing, then he would sit, then he would stand and deliver the sermon."