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

Muhammad bin Ya‘qub, from Ali bin Ibrahim, from his father, from Ibn Abi Umayr, from some of his companions: Abu Abdillah عليه السلام, peace be upon him, (said) regarding a man who prayed and did not know whether he had prayed two, three, or four rak‘ahs. Imam عليه السلام said: "Then he should stand and pray two rak‘ahs while standing and give the taslim, then pray two rak‘ahs while sitting and give the taslim. If he had actually prayed four rak‘ahs, then the two rak‘ahs are counted as recommended (nafilah), and if not, then the four rak‘ahs are completed."