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 Hammad bin ‘Isa, from Hariz, from Zurarah, from one of them (Imam عليه السلام), peace be upon them both, who said: I said to Him عليه السلام: “A man does not know whether he prayed one rak‘ah or two.” Imam عليه السلام said: "He should repeat (the prayer)." I asked: “A man does not know whether he prayed two rak‘ahs or three.” Imam عليه السلام said: "If the doubt enters after he has already begun the third rak‘ah, then he should continue in the third, then pray the next rak‘ah (the fourth), and there is nothing upon him, and he should give the taslim."