Sunan an Nasai
1574
Sahih+2

It was narrated that Simak said:"I asked Jabir: 'Did the Messenger of Allah (ﷺ) deliver the Khutbah standing?' He said: 'The Messenger of Allah (ﷺ) used to deliver the Khutbah standing, then he would sit for a while, then stand up again

SahihAbu Ghuddah
SahihAl-Albani