The Messenger of Allah ﷺ used to deliver the sermon standing, then sit down, then stand again and deliver the sermon standing. Whoever told you that he delivered the sermon sitting has lied, for by Allah, I prayed with him more than two thousand prayers.
أن رسول الله ﷺ كان يخطب قائما، ثم يجلس، ثم يقوم فيخطب قائما، فمن نبأك أنه كان يخطب جالسا فقد كذب، فقد والله صليت معه أكثر من ألفي صلاة.
Muslim narrated it in the Book of Jumuah (862), from Yahya bin Yahya, who said, Abu Khaythama informed us, from Simak, who said, Jabir bin Samura told me, and he mentioned it.His saying, "than a thousand prayers," means the obligatory prayers.