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

From him, from Ibn Abi Umayr, from Abdullah ibn Bukayr, from Ubayd ibn Zurarah, who said: I asked Abu Abdillah عليه السلام, peace be upon him, about a man who performs the morning prayer as one rak‘ah, recites the tashahhud, then finishes, leaves, and goes about, and afterward remembers that he had only prayed one rak‘ah. Imam عليه السلام said: "He should add one rak‘ah to it."