وَعَنْ جَابِرِ بْنِ سَمُرَةَ رَضِيَ اَللَّهُ عَنْهُمَا
«أَنَّ اَلنَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ كَانَ يَخْطُبُ قَائِمًا، ثُمَّ يَجْلِسُ، ثُمَّ يَقُومُ فَيَخْطُبُ قَائِمًا، فَمَنْ أَنْبَأَكَ أَنَّهُ كَانَ يَخْطُبُ جَالِسًا، فَقَدْ كَذَبَ»
The Prophet (sallallahu alayhi wa sallam) used to deliver the Khutbah (religious talk, sermon) while standing. He (sallallahu alayhi wa sallam) would then sit down and then stand up and deliver the Khutbah in a standing posture. So, whoever informs you that he delivered the Khutbah (religious talk, sermon) while sitting has definitely told a lie.
Bulugh al-Maram 542, Book 2, The Book of Prayer, Hadith 363. 1 min read