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