Musnad Ahmad 33:93

Waki’ narrated to us, al-A’mash narrated to us, from Abu Sufyan, from Jabir, who said: The Prophet, peace and blessings of Allah be upon him, fell from a horse onto the trunk of a palm tree, and his foot was dislocated. We entered upon him to visit him, and we found him praying, so we prayed with his prayer while we were standing. When he finished praying, he said: "The Imam was only appointed to be followed. If he prays standing, then pray standing, and if he prays sitting, then pray sitting. And do not stand while he is sitting, as the people of Persia do with their dignitaries."