Musnad Ahmad 788:111

Isma'il narrated to us, Sa'id narrated to us, from Qatadah, from Yunus ibn Jubayr, from Hattan ibn 'Abdullah al-Raqashi, from Abu Musa al-Ash'ari, who said: The Messenger of Allah (peace and blessings of Allah be upon him) taught us our prayer and our Sunnah. He said: "The Imam is only to be followed. So when he says 'Allahu Akbar' (God is the Greatest), then say 'Allahu Akbar'. And when he says: {Not of those who have evoked [Your] anger or of those who are astray} [Al-Fatiha: 7], then say: 'Amin' (Amen); Allah the Exalted will answer you. And when he bows, then bow; and when he rises, then rise. And when he says: 'Sami' Allahu liman hamidah' (Allah hears those who praise Him), then say: 'Rabbana wa lakal-hamd' (Our Lord, and to You is the praise); Allah will hear you. And when he prostrates, then prostrate; and when he rises, then rise; for the Imam prostrates before you and rises before you." The Messenger of Allah (peace and blessings of Allah be upon him) said: "That is for that."