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