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

And what was narrated by Sa‘d bin Abdillah, from Muhammad bin al-Husayn, from Ahmad bin Muhammad bin Abi Nasr, from Abd al-Karim bin ‘Amr, from Abdillah bin Abi Ya‘fur, who said: I asked Abu Abdillah عليه السلام, peace be upon him, about a man who does not know whether he prayed two rak‘ahs or one. Imam عليه السلام said: "He completes (the prayer) with one rak‘ah."