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

And from him, from al-Hajjal, from Abdillah عليه السلام, from ‘Ubayd: Abu Abdillah عليه السلام, peace be upon him, said regarding a man who prayed one rak‘ah of Fajr, then went away and came back after morning had begun and remembered that he had prayed only one rak‘ah. Imam عليه السلام said: "He should add one rak‘ah to it."