Musannaf Ibn Abi Shaybah

Hazrat Aisha (may Allah be pleased with her) narrates that when the Prophet Muhammad (peace and blessings be upon him) fell ill, some of the companions came to visit him. The Prophet (peace and blessings be upon him) prayed sitting, while those people prayed standing behind him in his congregation. The Prophet (peace and blessings be upon him) gestured to them to sit down, so they sat. When the Prophet (peace and blessings be upon him) finished the prayer, he said: "The Imam is appointed so that he may be followed. So when he bows, you bow; and when he raises his head, you raise your head; and when he prays sitting, you pray sitting."