Sahih al-Bukhari 920
صحيح البخاري#920SahihNarrated Ibn `Umar:The Prophet (ﷺ) used to deliver the Khutba while standing and then he would sit, then stand again as you do now-a-days
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih al-Bukhari is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 11:44