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

From him, from Hamdawayh, from Muhammad ibn al-Husayn, from al-Hasan ibn Mahbub, from Abdul-Aziz, from Abdullah ibn Abi Ya‘fur, from Abu Abdillah عليه السلام, peace be upon him, who said: I asked Imam عليه السلام about a man who stood for an obligatory prayer and prayed one rak‘ah while intending it to be a supererogatory prayer. Imam عليه السلام said: "It is according to what you stood for and intended." And Imam عليه السلام said: "If you stood while intending an obligatory prayer, and then doubt entered upon you afterward, then you are in the obligatory prayer according to what you stood for. But if you began it while intending it as a supererogatory prayer, and then afterward you intend it as obligatory, then you are in the supererogatory prayer. Indeed, what is counted for the servant from his prayer is what he began with at the start of his prayer."