(10) باب ذِكْرِ الْخُطْبَتَيْنِ قَبْلَ الصَّلاَةِ وَمَا فِيهِمَا مِنَ الْجَلْسَةِ
Ibn 'Umar said that the Messenger of Allah (ﷺ) used to deliver the sermon on Friday while standing. He would then sit and then stand (for the second sermon) as they (the Muslims) do nowadays.
Reference: Sahih Muslim 861
In-book reference: Book 7, Hadith 44
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 1874