What is the ruling on fasting the day of Arafah with a prior intention of making up for missed fasts (qada'), then forgetting the intention of qada' on the day of Arafah and fasting it with the intention of a voluntary (nafl) fast? Is it counted as qada' or only as nafl?

Intention is a condition for the validity of fasting and all acts of worship, as the Prophet, peace and blessings be upon him, said: "Actions are but by intentions, and every person will have what he intended." For obligatory fasting, such as making up for missed Ramadan (definition) fasts, the intention must be formed during the night before the dawn breaks, based on the Prophet's saying, peace and blessings be upon him: "Whoever does not gather the intention to fast before dawn, then there is no fast for him." The intention must be firm and specific. So, if a person intends to fast the day of Arafah voluntarily, it is not permissible to count this fast as a make-up fast, because the intention formed overnight was for a voluntary act, not for an obligatory one.