Is it permissible to combine the intention of making up for missed Ramadan fasts with the intention of fasting the six days of Shawwal, and thereby obtain the reward for fasting the six days?

It appears that fasting the six days of Shawwal with the intention of making up missed fasts is not permissible, because both the fast of Ramadan and the fast of the six days are acts of worship intended for their own sake, and thus combining intentions in them is not valid. The fast of the six days is like a supererogatory fast after Ramadan; it requires a specific intention and is not subsumed under the make-up fast. Ibn Baz stated that the fast of the six days requires a specific intention on specific days. This supports the idea that the reward for the six days is not obtained by someone who fasts them with the intention of making up missed fasts.

This matter has been debated, and scholars have differed on it. Some have held that the reward for the six days is obtained by someone who fasts make-up days in Shawwal, even if they did not originally intend the six days. Some scholars have held that whoever fasts with both intentions will not receive the reward for either the make-up fast or the fast of the six days.

It should be noted that calling the six days of Shawwal "Al-Ayyam Al-Bid" (the White Days) has no basis; the White Days are the thirteenth, fourteenth, and fifteenth of the lunar month.