كتاب الجمعة
7  
The Book of Prayer - Friday
(10)
Chapter: The two khutbah before the prayer, and sitting briefly in between them
(10)
باب ذِكْرِ الْخُطْبَتَيْنِ قَبْلَ الصَّلاَةِ وَمَا فِيهِمَا مِنَ الْجَلْسَةِ ‏
Sahih Muslim 862 b

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.

وَحَدَّثَنَا يَحْيَى بْنُ يَحْيَى، أَخْبَرَنَا أَبُو خَيْثَمَةَ، عَنْ سِمَاكٍ، قَالَ أَنْبَأَنِي جَابِرُ بْنُ، سَمُرَةَ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم كَانَ يَخْطُبُ قَائِمًا ثُمَّ يَجْلِسُ ثُمَّ يَقُومُ فَيَخْطُبُ قَائِمًا فَمَنْ نَبَّأَكَ أَنَّهُ كَانَ يَخْطُبُ جَالِسًا فَقَدْ كَذَبَ فَقَدْ وَاللَّهِ صَلَّيْتُ مَعَهُ أَكْثَرَ مِنْ أَلْفَىْ صَلاَةٍ.
Reference : Sahih Muslim 862b
In-book reference : Book 7, Hadith 46
USC-MSA web (English) reference : Book 4, Hadith 1876
  (deprecated numbering scheme)