Sunan Ibn Majah, Chapter: The Chapters of Establishing the Prayer and the Sunnah Regarding Them

"The Messenger of Allah (saw) fell ill, and we prayed behind him while he was sitting down, and Abu Bakr was saying the Takbir so that the people could hear them. He turned to us and saw us standing, so he gestured to us to sit down. When he had said the Salam, he said: 'You were about to do the action of the Persians and Romans, who remain standing while their kings are seated. Do not do that. Follow the lead of your Imam; if he prays standing, then pray standing, and if he prays sitting down, then pray sitting down.'" Daif