Sahih Ibn Khuzaymah

Umm al-Mu’minin Sayyidah Aisha (may Allah be pleased with her) narrates that some people came to the Prophet Muhammad (peace be upon him) while he was ill. He (peace be upon him) led them in prayer while sitting, and they began the prayer standing. He gestured to them to sit down. (After finishing the prayer) the Prophet (peace be upon him) said: “Indeed, the imam is appointed so that he may be followed, so when he leads the prayer sitting, you should also pray sitting, and when he leads the prayer standing, you should also pray standing, and when he bows (in ruku‘), you should also bow, and when he prostrates, you should also prostrate, and when he raises his head, you should also raise your heads.”