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

Al-Husayn bin Sa‘id, from Fadalah, from al-Husayn bin ‘Uthman, from Sama‘ah, from Abu Basir, who said: I asked Him عليه السلام, peace be upon him, about a man who prayed and did not know whether he was in the third rak‘ah or the fourth. Imam عليه السلام said: "Whatever his assumption inclines toward, if he thinks he is in the third and there is still something in his heart about the fourth, then he should conclude (the prayer) between himself and his Lord عز وجل, then pray two rak‘ahs in which he recites the Opening of the Book (Surah al-Fatiha) in both."