The Messenger of Allah ﷺ used to deliver the sermon standing, then he would sit down, then stand and preach standing. If anyone tells you he preached sitting, he is lying. I swear by Allah that I offered along with more than two thousand prayers.  

أَنَّ رَسُولَ اللَّهِ ﷺ كَانَ يَخْطُبُ قَائِمًا ثُمَّ يَجْلِسُ ثُمَّ يَقُومُ فَيَخْطُبُ قَائِمًا فَمَنْ حَدَّثَكَ أَنَّهُ كَانَ يَخْطُبُ جَالِسًا فَقَدْ كَذَبَ فَقَالَ فَقَدْ وَاللَّهِ صَلَّيْتُ مَعَهُ أَكْثَرَ مِنْ أَلْفَىْ صَلاَةٍ  

The Messenger of Allah ﷺ gave two sermons between which he sat, recited the Quran and gave the people an exhortation.  

كَانَ لِرَسُولِ اللَّهِ ﷺ خُطْبَتَانِ كَانَ يَجْلِسُ بَيْنَهُمَا يَقْرَأُ الْقُرْآنَ وَيُذَكِّرُ النَّاسَ  

I saw the Prophet ﷺ would deliver the sermon standing, then sit down without saying anything. The narrator then reported the tradition in full.  

رَأَيْتُ النَّبِيَّ ﷺ يَخْطُبُ قَائِمًا ثُمَّ يَقْعُدُ قَعْدَةً لاَ يَتَكَلَّمُ وَسَاقَ الْحَدِيثَ