Is it permissible to make an intention of Qadha for fasts that were missed in Ramadhan and also for the 6 Sunna fasts of Shawwal?
In the name of Allah, Most Compassionate, Most Merciful,
Sayyiduna Abu Ayyub al-Ansari (may Allah be pleased with him) relates that the Messenger of Allah (Allah bless him & give him peace) said: “Whoever fasts Ramadhan, then follows it with six days from Shawwal, it is like they fasted the entire year.” (Sahih Muslim no: 1164)
Based on this and other Hadiths, the majority of the jurists (fuqaha), including the Hanafi, Shafi’i and Hanbali Schools, hold that it is recommended to fast six days of the month of Shawwal. (See: Radd al-Muhtar, Mughni al-Muhtaj and Kashshaf al-Qina’)
As for combining two intentions, i.e. making-up missed Ramadhan fasts (qadha) and the six fasts of Shawwal, one should principally make an intention of Qadha. Thereafter, as a ‘secondary’ intention, one may also keep in mind that it is the month of Shawwal. By doing so, it is hoped from the mercy of Allah that along with the missed fasts being made-up, one will receive the reward of fasting in Shawwal. But, it is important that the main and predominant intention is of making-up the missed fasts.
However, it is better that one makes up for the missed fasts separately from fasting the six days of Shawwal, since some scholars are of the view that one does not receive the reward of optional (nafl) fasting when combined with make-up (qadha) fasts. (See: Fatawa Darul Uloom Deoband 6/495)
One of the explanations given for fasting the six days of Shawwal being equivalent to fasting the entire year is that a good deed is multiplied by ten, and as such, fasting the whole of Ramadhan would count as fasting ten months, and fasting the six days of Shawwal, multiplied by ten, would equal to two months. Hence, it would be as though one has fasted the entire year. (See: Ibn Rajab al-Hanbali’s Lata’if al-Ma’arif 272)
In fact, this has been clearly mentioned in a Hadith related by Thawban (may Allah be pleased with him) that the Messenger of Allah (Allah bless him & give him peace) said: “Fasting Ramadhan is like fasting ten months, and fasting the six days is like fasting two months, so that amounts to fasting the entire year.” (Recorded by Imam Ahmad in his Musnad, Ibn Hibban in his Sahih and Imam Nasa’i in his Sunan, taken from Ibn Rajab’s Lata’if 272)
So, in order to receive this specific reward of fasting the entire year, one would have to fast the whole month of Ramadhan (regardless of whether it consists of 29 or 30 days), and then follow it up with fasting the six days of Shawwal. For this reason, we see in the Hadith recorded by Muslim (quoted in the beginning of this answer) that the Messenger of Allah (Allah bless him & give him peace) said: “Whoever fasts Ramadhan, then follows it with…” i.e. one would need to first complete the fasts of Ramadhan before fasting the six days of Shawwal to attain this particular reward.
As such, in conclusion, it is without doubt better to make-up for the missed Ramadhan fasts first and then observe the six fasts of Shawwal in order to obtain maximum rewards and the specific reward of fasting the entire year. However, it is permitted to combine make-up fasts with the six fasts of Shawwal provided the principle intention (niyya) is of Qadha fasts.