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 ﷺ 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.