(27) باب الْخُطْبَةِ قَائِمًا
Narrated Ibn `Umar:
The Prophet (p.b.u.h) used to deliver the Khutba while standing and then he would sit, then stand again as you do now-a-days.
Reference: Sahih al-Bukhari 920
In-book reference: Book 11, Hadith 44
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 2, Book 13, Hadith 43