Musnad Abu Ya'la al-Mawsili 160:177

'Abd al-Rahman ibn Salih narrated to us, 'Abd al-Rahim narrated to us, 'Abd Allah ibn Sa'id narrated to us, from his father, from his grandfather, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "The Imam was only appointed to be followed. So when he says the takbir, say the takbir; when he bows, bow; when he prostrates, prostrate; when he rises, rise; when he sits, sit; and when he stands, stand. The Imam is a shield and a guarantor for the prayer of the people. If he performs it at its time and establishes its limits—I think he said—he shall have his reward and the like of their rewards, without their rewards being diminished in the slightest. And whoever does not perform it at its time and does not establish its limits, the burden of it and the burden of their sins shall be upon him, and there shall be nothing upon them."