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

And what was narrated by Muhammad bin Ahmad bin Yahya, from al-Sindi bin al-Rabi‘, from al-Hasan bin Mahbub, from Abd al-Rahman bin al-Hajjaj: Abu Ibrahim عليه السلام, peace be upon him, said regarding a man who does not know whether he prayed one rak‘ah or two. Imam عليه السلام said: "He builds upon the one rak‘ah."