بُلُوغُ الْمَرَامِ
Bulugh al-Maram 450
The Book of Prayer
وَعَنْ جَابِرِ بْنِ سَمُرَةَ رَضِيَ اَللَّهُ عَنْهُمَا, { أَنَّ اَلنَّبِيَّ - صلى الله عليه وسلم -كَانَ يَخْطُبُ قَائِمًا, ثُمَّ يَجْلِسُ, ثُمَّ يَقُومُ فَيَخْطُبُ قَائِمًا, فَمَنْ أَنْبَأَك َ [1] أَنَّهُ كَانَ يَخْطُبُ جَالِسًا, فَقَدْ كَذَبَ } أَخْرَجَهُ مُسْلِم ٌ [2] .
Narrated Jabir bin Samurah (RA): The Prophet (ﷺ) used to deliver the Khutbah (religious talk, sermon) while standing. He (ﷺ) would then sit down and then stand up and deliver the Khutbah in a standing posture. So, whoever informs you that he [the Prophet (ﷺ)] delivered the Khutbah (religious talk, sermon) while sitting has definitely told a lie. [Reported by Muslim].
Sunnah.com reference: Book 2, Hadith 367