Musnad Ahmad 33:475

Yunus and Hujayn narrated to us, Layth narrated to us, from Abu al-Zubayr, from Jabir, who said: The Messenger of Allah (peace and blessings of Allah be upon him) became ill, so we prayed behind him while he was sitting, and Abu Bakr was saying the takbir, making the people hear his takbir. He turned to us and saw us standing, so he signaled to us, and we sat down and prayed by his prayer while sitting. When he finished the prayer, he said: "You were about just now to do the action of Persia and Rome; they stand for their kings while they are sitting. Do not do that. Follow your imams; if he prays standing, then pray standing, and if he prays sitting, then pray sitting."