Musnad Ahmad 907:46

Hashim ibn al-Qasim narrated to us, Zuhayr narrated to us, Simak ibn Harb narrated to us, he said: Jabir ibn Samurah informed me, "That the Messenger of Allah, may Allah bless him and grant him peace, used to deliver the sermon while standing upon the pulpit, then he would sit, then he would stand and deliver the sermon while standing. So whoever informs you that he used to deliver the sermon while sitting has lied. For by Allah, I have prayed with him more than two thousand prayers."