Chapter on the Virtue of Mosques and Prayer Within Them, and the Virtue of Congregational Prayer and Its Rulings

From him, from al-Barqi from Ibn Faddal who said: I wrote to Abu al-Hasan al-Ridha عليه السلام, peace be upon him, regarding a man who was behind an imam (of prayer), following him, and he bowed before the imam (of prayer) thinking that the imam (of prayer) had bowed. When he bowed, he saw that the imam (of prayer) had not bowed, so he raised his head, then repeated the bowing with the imam (of prayer). Does this invalidate his prayer, or is that rak‘ah valid? Imam عليه السلام wrote: “He completes his prayer, and what he did does not invalidate his prayer.”