Praise be to Allah.
As we celebrate Eid and bask in the festivities, there is one special sunnah of fasting which only presents in this joyous month that we should not miss - it is the Sunnah of fasting the six days in Shawwal. Prophet Muhammad s.a.w. mentioned:
“Fasting in Ramadan and following it with six days in Shawwal is like continual fasting.”
[Hadith from Imam Muslim]
Scholars have explained that the reward of actions is multiplied by at least ten-fold. Based on the statement above, fasting in Ramadan for thirty days is equivalent to fasting three hundred days, and the six days of Shawwal is akin to fasting for sixty days. Our beloved Prophet Muhammad s.a.w. himself stated this explicitly:
“Fasting Ramadan is like fasting ten months, and fasting six days [of Shawwal] is like fasting for two months. That is like fasting a full year.”
[Hadith from Ahmad & Nasai]
It is encouraged for Muslims to fast six days of Shawwal consecutively as it provides greater reward and virtue in it. Based on the hadith narrated by al-Tabarani and others reported that the Prophet s.a.w. said:
“Fasting six consecutive days after Eidulfitri is like fasting the entire year.”
However, if someone is unable to do so, it is still valid for as long as it is within the month of Shawwal.
It is valid to combine the intention of making up missed fasts during Ramadan and the Sunnah of fasting the six days of Shawwal, though performing both separately have a greater reward. This is as mentioned by Al-Hafidz As-Suyuthi in Al-Asybah wa al-Nadhair, where he cited this to the position of al-Bariziy. However, you would need to make the intention that this fast is primarily intended to make up for the missed fast, and the fast of six days of Shawwal is only a supplementary one.
It is for this reason that Imam Ar-Ramli in his book Nihayatul Minhaj ila Syarh al-Minhaj stated that while it is permissible to combine both fasts, example, missed fasts and the fast of six days of Shawwal, the reward would be as comparable with the one who fasts separately where they will naturally gain more rewards.
On the other hand, Scholar like Sheikh Ali Gom’ah who opines that the hadith of fasting six days of Shawwal is general and for as long as you have fasted 6 days in Shawwal whether, for the primary purpose of repaying missed fasts or another sunnah fasting, you have obtained the reward of fasting six days of Shawwal.
Regardless of any views that we opt, we should accept others of adopting a different opinion on this matter. Any opinions that based on scholarly Ijtihad has goodness in it, Insya Allah.
Allah knows best.