(19) The Book of the Prayer for the Two 'Eids
كتاب صلاة العيدين
(18) Chapter: Imam standing during the Khutbah
(18) باب قِيَامِ الإِمَامِ فِي الْخُطْبَةِ
Sunan an-Nasa'i 1574 
It was narrated that Simak said:
"I asked Jabir: 'Did the Messenger of Allah (ﷺ) deliver the Khutbah standing?' He said: 'The Messenger of Allah (ﷺ) used to deliver the Khutbah standing, then he would sit for a while, then stand up again.'"
أَخْبَرَنَا إِسْمَاعِيلُ بْنُ مَسْعُودٍ، قَالَ حَدَّثَنَا خَالِدٌ، قَالَ حَدَّثَنَا شُعْبَةُ، عَنْ سِمَاكٍ، قَالَ سَأَلْتُ جَابِرًا أَكَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يَخْطُبُ قَائِمًا قَالَ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يَخْطُبُ قَائِمًا ثُمَّ يَقْعُدُ قَعْدَةً ثُمَّ يَقُومُ ‏.‏
Grade :  Sahih (Darussalam)
Reference : Sunan an-Nasa'i 1574
In-book reference : Book 19, Hadith 19
English translation : Vol. 2, Book 19, Hadith 1575