Musnad Imam Shafi'i

Narrated from Aisha (may Allah be pleased with her), she states: The Messenger of Allah (ﷺ) performed prayer in my house, and he (ﷺ) was ill. He (ﷺ) prayed sitting, and the people behind him prayed standing, so he (ﷺ) gestured to them to sit down. When he (ﷺ) finished the prayer, he said: “The imam is appointed so that he may be followed. When he bows, you should bow; and when he raises his head, you should raise your heads; and when he prays sitting, you should also pray sitting.”