Chapter on the Rulings on Forgetfulness (Ahkam al-Sahw)

As for what was narrated by Sa‘d from Muhammad ibn al-Husayn from Ja‘far ibn Bashir from Hammad ibn Uthman from Ubayd ibn Zurarah, who said: I asked Abu Abdillah عليه السلام, peace be upon him, about a man who performed one rak‘ah of the morning prayer, then finished and went out to attend to his needs, then remembered that he had only prayed one rak‘ah. Imam عليه السلام said: "Then let him complete what remains."