Musannaf Ibn Abi Shaybah

It is narrated from Hazrat Zuhri that I heard Anas bin Malik (may Allah be pleased with him) say that the Noble Prophet (peace and blessings be upon him and his family) fell from a horse and his right side was injured. We went to check on him, and during this time the time for prayer came. The Prophet (peace and blessings be upon him and his family) led us in prayer while sitting, and we also prayed sitting in his following. So when the prayer was completed, the Prophet (peace and blessings be upon him and his family) said: The Imam is appointed so that he may be followed. So when the Imam says the takbeer, you should say the takbeer. And when he bows (in ruku‘), you should bow. And when the Imam prostrates, you should prostrate. And when the Imam raises his head, you should raise your heads. And when the Imam says "Sami‘a Allahu liman hamidah," you should say "Allahumma Rabbana wa lakal hamd." And if the Imam prays sitting, then you all pray sitting.