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

As for what was narrated by Muhammad bin Ahmad bin Yahya, from Muhammad bin al-Husayn, from Ja‘far, from Hammad bin ‘Isa, from ‘Ubayd bin Zurarah, from Abu Abdillah عليه السلام, peace be upon him, who said: I asked Him عليه السلام about a man who does not know whether he prayed two rak‘ahs or three. Imam عليه السلام said: "He must repeat (the prayer)." I asked: “Is it not said that a jurist does not repeat the prayer?” Imam عليه السلام said: "That is only in the case of doubt between three and four (rak‘ahs)."