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

Muhammad ibn al-Hasan said: “The interpretation of this report is that we should apply it to supererogatory (nafilah) prayers, because the ruling for supererogatory prayers is that they are to be built upon the lesser number out of precaution, as we have explained. As for obligatory (fard) prayers, they are to be built upon the greater number, and then completed after finishing the prayer, as we have clarified.”