Sahih Muslim 861
Sahih Muslim · Book 7: The Book of Prayer - Friday · The two khutbah before the prayer, and sitting briefly in between them
وَحَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ عُمَرَ الْقَوَارِيرِيُّ ، وَأَبُو كَامِلٍ الْجَحْدَرِيُّ جَمِيعًا عَنْ خَالِدٍ ، - قَالَ أَبُو كَامِلٍ حَدَّثَنَا خَالِدُ بْنُ الْحَارِثِ، - حَدَّثَنَا عُبَيْدُ اللَّهِ ، عَنْ نَافِعٍ ، عَنِ ابْنِ عُمَرَ ، قَالَ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يَخْطُبُ يَوْمَ الْجُمُعَةِ قَائِمًا ثُمَّ يَجْلِسُ ثُمَّ يَقُومُ . قَالَ كَمَا يَفْعَلُونَ الْيَوْمَ .
Ibn 'Umar said that the Messenger of Allah (ﷺ) used to deliver the sermon on Friday while standing. He would then sit and then stand (for the second sermon) as they (the Muslims) do nowadays.