From Sahih Muslim (Hadith 1996):
Jabir b. Samura said that the Messenger of Allah (ﷺ) used to deliver the sermon while standing. He would then sit down and then stand up and address in a standing posture; and whoever informed you that he (the Holy Prophet) delivered the sermon while sitting told a lie. By Allah. I prayed with him more than two thousand times
Reference: Muslim 1996