Answered by Ustadha Shazia Ahmad
Fasting six days of Shawwal is like fasting one year. But does that mean that fasting 3 days of Shawwal is like fasting half the year? What if one fasts less than 6 days of Shawwal?
There is no such hadith that says fasting only six days of Shawwal is like fasting the whole year. One must first fast Ramadan, as well.
The hadith that has come of the Prophet, Allah’s peace and blessings be upon him, is: “Whoever fasts Ramadan, then follows it with six from Shawwal, it is as though he fasted all of time.”
All of time implies the whole year, for if one did this every year, it would be all of time.
It says in Nuzhat al-Mutaqin, the commentary on Imam Nawawi‘s Riyad al-Salihin (2/132), that, “Whoever fasts Ramadan and follows it up with six from Shawwal, it is as though he has fasted all of time, because each day is equivalent to ten days, ‘Whoever performs a good deed, he will have ten like it’. So Ramadan would count as ten months and six days multiplied by ten would equal two months. It is best that one fast the six altogether and directly after the day of `Eid.”
According to the above, our understanding is that the reward of one’s fasting is according to how much one fasts. If Allah accepts our fasts, and then counts each one as equivalent to ten, it will be considered as such, from His limitless mercy. Although a hadith has not come to us about fasting less than six in Shawwal, one can assume that his worship would not be complete unless he performed the full number.
And Allah knows best.