The Messenger of Allah ﷺ used to deliver the sermon standing, except that he would sit briefly and then stand up again.
حدثنا محمد بن بشار ومحمد بن الوليد قالا حدثنا محمد بن جعفر حدثنا شعبة عن سماك بن حرب قال سمعت
كان رسول الله ﷺ يخطب قائما غير أنه كان يقعد قعدة ثم يقوم
Muslim narrated it in "The Friday Prayer," chapter on mentioning the two sermons before the prayer and the sitting between them, hadith no. 862, and Nasai narrated it in "The Prayer of the Two Eids," chapter on the imam standing during the sermon (3/186, hadith no. 1575, from the hadith of Shubah), both from the hadith of Simak, with similar wording.