Musnad Ahmad 1153:1600

Yahya narrated to us, from Hisham ibn Urwah, who said: My father narrated to me, saying: Aisha informed me that the Messenger of Allah (peace and blessings of Allah be upon him) had people enter upon him during his illness to visit him. He prayed with them while sitting, and they began to pray while standing. He signaled to them to sit. When he finished, he said: "The Imam is only appointed to be followed. So when he bows, then bow; and when he rises, then rise; and if he prays sitting, then pray sitting."