Chapter on Rulings on Forgetfulness (Sahw) in Prayer and What Requires Repeating of Prayer

Muhammad bin Ahmad bin Yahya, from Ya‘qub bin Yazid, from Ali bin al-Nu‘man, from al-Husayn bin Abi al-‘Ala’, from Abu Abdillah عليه السلام, peace be upon him, who said: I (narrator) said: "I came to the imam (of prayer) while he had preceded me by one rak‘ah in the Fajr prayer. When he completed the prayer with the taslim, it occurred in my heart that I had completed (my prayer), so I remained engaged in the remembrance of Allah سبحانه وتعالى until the sun rose. When the sun rose, I stood up and remembered that the Imam had preceded me by one rak‘ah." Imam عليه السلام said: "If you were still in your place, then complete it with one rak‘ah; but if you had already left, then you must repeat the prayer."