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." (PT)
App reference (PT): Book 14, Hadith 54 (PT)
English reference (PT): Vol. 2, Book 14, Hadith 1418 (PT)
Arabic reference (PT): Book 14, Hadith 1428 (PT)