Musnad Ahmad 1153:1131

I read to Abd al-Rahman: Malik, from Hisham ibn Urwah, from his father, from Aisha, the wife of the Prophet, may God bless him and grant him peace, that she said: The Messenger of God, may God bless him and grant him peace, prayed in his house while he was ill, so he prayed sitting, and a group of people prayed behind him standing. He signaled to them to sit down. 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 when he prays sitting, then pray sitting."