Musnad Ahmad 907:26

Husayn ibn Muhammad narrated to us, Sulayman ibn Qarm 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 while standing. So whoever tells you that he saw him ever delivering the sermon while not standing, he has lied. However, he would sometimes come out and see the people in small numbers, so he would sit down, then they would gather, then he would stand and deliver the sermon."