Musannaf Ibn Abi Shaybah

Hazrat Aisha (may Allah be pleased with her) narrates that once the Prophet (peace and blessings be upon him) became ill, so some of his companions came to visit him. Meanwhile, the time for prayer arrived, so he led them in prayer while sitting. They began to pray standing, so the Prophet (peace and blessings be upon him) gestured to them to pray sitting. Thus, they sat down. When the Prophet (peace and blessings be upon him) completed the prayer, he said: "The Imam is appointed so that he may be followed; when he bows, you should also bow; when he raises his head from bowing, you should also raise your heads; and when he prays sitting, you should also pray sitting."