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

Muhammad ibn Mas‘ud al-‘Ayyashi from Ja‘far ibn Ahmad from Ali ibn al-Hasan from Muhammad ibn Isa from Yunus from Mu‘awiyah, who said: I asked Abu Abdillah عليه السلام, peace be upon him, about a man who stood for an obligatory prayer but due to forgetfulness thought it was a supererogatory one, or was in a supererogatory prayer and thought it was an obligatory one. Imam عليه السلام said: "It is according to what he began the prayer upon."