Muhammad ibn Ahmad ibn Yahya from Ahmad ibn al-Hasan from Amr ibn Saeed from Musaddiq ibn Sadaqah from Ammar al-Sabati from Abu Abdillah عليه السلام, peace be upon him, who said: I asked Imam al-Sadiq عليه السلام about a man who catches imam (of prayer) while he is sitting and reciting tashahhud, and there is no one behind him except a man on his right. Imam عليه السلام said: “The imam (of prayer) should not move forward, and the man should not move backward, but the one who enters to pray with him should sit behind the imam (of prayer). When the imam (of prayer) gives the salam, the man should stand and complete his prayer.”