The Messenger of Allah (may Allah's peace and blessings be upon him) used to deliver two sermons while standing, and he would separate them by sitting
Sahih/Authentic with its two versionsعن عبد الله بن عُمر بن الخطاب رضي الله عنهما قال: «كان النبي صلى الله عليه وسلم يخطب خطبتين يقعد بينهما» وفي رواية لجابر - رضي الله عنه-: «كان رسول الله صلى الله عليه وسلم يَخْطُبُ خُطْبَتَيْنِ وهو قائم، يفصل بينهما بجلوس».
‘Abdullāh ibn ‘Umar ibn al-Khattāb (may Allah be pleased with him) reported: The Prophet (may Allah's peace and blessings be upon him) used to deliver two sermons between which he used to sit. Another narration reported by Jābir (may Allah be pleased with him) reads: "The Messenger of Allah (may Allah's peace and blessings be upon him) used to deliver two sermons while standing, and he would separate them by sitting."
Explanation
Friday is a large all-inclusive gathering for the people of a town, and that is why, out of his wisdom, the Prophet (may Allah's peace and blessings be upon him) used to deliver two sermons on Friday to direct the people towards good and warn them against evil. He used to give the two sermons while standing on the pulpit in order to be more eloquent in teaching and reminding them, in addition to the fact that standing displays the strength and splendor of Islam. When he concluded the first sermon, he would sit for a short while in order to rest, thereby separating the first sermon from the second. Then he would stand and give the second sermon; (he did this) so that the orator would not tire and the listeners would not become bored.
Source · Al-Bayhaqi - Narrated by Bukhari & Muslim
From the Hadith Encyclopedia (hadeethenc.com).