Musnad Ahmad 30:1894

Bahz narrated to us, Hammad ibn Salamah narrated to us, he said: Ya'la ibn 'Ata' informed us, from Abu 'Alqamah—and Abu 'Awanah al-Ansari said: from Abu Hurairah—that the Messenger of Allah (ﷺ) said: "Whoever obeys me has obeyed Allah, and whoever disobeys me has disobeyed Allah. Whoever obeys the commander has obeyed me, and whoever disobeys the commander has disobeyed me. The commander is a shield; so when he says 'Allahu Akbar' (God is the Greatest), say 'Allahu Akbar'. When he bows, bow. And when he says, 'Sami'a Allahu liman hamidah' (Allah hears those who praise Him), say, 'Allahumma Rabbana laka al-hamd' (O Allah, our Lord, to You is the praise). For if that coincides with the saying of the angels, you will be forgiven. And if he prays sitting, then pray sitting."