(10) باب ذِكْرِ الْخُطْبَتَيْنِ قَبْلَ الصَّلاَةِ وَمَا فِيهِمَا مِنَ الْجَلْسَةِ
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: Sahih Muslim 862 b
In-book reference: Book 7, Hadith 46
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 1876