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

Al-Husayn bin Sa‘id, from Hammad, from Hariz, from Muhammad bin Muslim, who said: I asked Abu Abdillah عليه السلام, peace be upon him, about a man who prayed two rak‘ahs and does not know whether they were two or four. Imam عليه السلام said: "He should give the taslim, then stand and pray two rak‘ahs, reciting the Opening of the Book (Surah al-Fatiha) in them, perform the tashahhud, and then conclude (the prayer), and nothing is required of him."