أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا أَبُو عَوَانَةَ، عَنْ سِمَاكٍ، عَنْ جَابِرِ بْنِ سَمُرَةَ، قَالَ رَأَيْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَخْطُبُ قَائِمًا ثُمَّ يَقْعُدُ قَعْدَةً لاَ يَتَكَلَّمُ فِيهَا ثُمَّ قَامَ فَخَطَبَ خُطْبَةً أُخْرَى فَمَنْ خَبَّرَكَ أَنَّ النَّبِيَّ صلى الله عليه وسلم خَطَبَ قَاعِدًا فَلاَ تُصَدِّقْهُ ‏.‏

It was narrated that Jabir bin Samurah said: "I saw the Messenger of Allah (PBUH) delivering the Khutbah standing up, then he sat down for a while and did not speak, then he stood up and delivered another Khutbah. Whoever tells you that the Prophet (PBUH) delivered a Khutbah sitting do not believe him." Translation Not Available

Book reference: Sunan An Nisai Book 19 Hadith 1583
Web reference: Sunan An Nisai Volume 2 Book 19 Hadith 1584