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

Muhammad bin Ahmad bin Yahya, from Muhammad bin al-Husayn, from Muhammad bin Abdillah bin Hilal, from al-‘Ala’, from Muhammad bin Muslim, who said: I asked Abu Ja‘far عليه السلام about a man who, after finishing the Dhuhr prayer, becomes certain that he had prayed five rak‘ahs. Imam عليه السلام said: "And how did he become certain?" I said: “He knows it (to be certain).” Imam عليه السلام said: "If he knows that he had sat in the fourth rak‘ah, then his Dhuhr prayer is complete, and he should stand and add to the fifth rak‘ah another rak‘ah and two prostrations, so they will become two (extra) rak‘ahs as ‘nafilah’ (recommended prayer), and nothing is upon him."