The Messenger of Allah ﷺ used to deliver the Friday sermon standing, then he would sit, then stand again, just as you do today.
كان رسول الله ﷺ يخطب يوم الجمعة قائما، ثم يجلس، ثم يقوم، كما تفعلون اليوم.
Bukhari narrated it in Jumuah (920) and Muslim in Jumuah (861), both through routes from Khalid bin Harith, who said, "Ubaydullah narrated to us, from Nafi, from Ibn Umar," and he related the hadith. The wording given is Muslim's, and Bukhari's wording is the same except that he did not mention "the day of Jumuah."Abu Dawud (1092) recorded it through the route of Umari, from Nafi, with this wording, "The Prophet ﷺ used to deliver two sermons. He would sit when he ascended the pulpit until he finished," and I think he said, "the muezzin," "then he would stand and deliver the sermon, then he would sit and not speak, then he would stand again and deliver the sermon."