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

Ahmad bin Muhammad, from Muhammad bin Sahl, from his father, who said: I asked Abu al-Hasan عليه السلام, peace be upon him, about a man who does not know whether he prayed three rak‘ahs or two. Imam عليه السلام said: "He should build upon the lesser number and act according to what he is certain of. After finishing his prayer, he should recite a light tashahhud. This is to be done both at the beginning of the prayer and at its end."