It was narrated that Jabir bin Samurah said:
"I saw the Messenger of Allah (ﷺ) delivering the khutbah on Friday standing, then he sat briefly and did not speak, then he stood up and delivered a second khutbah. So whoever tells you that the Messenger of Allah (ﷺ) used to deliver the khutbah seated, he has lied."
App reference: Book 14, Hadith 54
English reference: Vol. 2, Book 14, Hadith 1418
Arabic reference: Book 14, Hadith 1428