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

Ali ibn Ibrahim from his father from Abdullah ibn al-Mughirah, who said: In the book of Hariz, he said: “I forgot that I was in an obligatory prayer until I had already bowed, and I had intended it as a supererogatory prayer.” He said: "It is according to what you stood for, if you stood with the intention of an obligatory prayer and then doubt entered upon you, then you are in the obligatory prayer. But if you began with the intention of a supererogatory prayer and then intended it to be obligatory, then you are in the supererogatory prayer. And if you began in an obligatory prayer and then remembered a supererogatory prayer that was due upon you, then proceed with the obligatory prayer."