Hazrat Abu Ayoob Ansari (R) reports that Rasulullah (Sallallahu Alaihi Wasallam) said: “Whoever completes the fasts of Ramadan then adds to them the fasts of six days in the month of Shawwal, it will carry the reward of fasting for the whole year.” (Sahih Muslim)
Considering that there is at least ten-fold reward for every virtue, the fasts of Ramadan will equate to fasting for 300 days - ten months. Fasting for six days in Shawwal will be considered as fasting for 60 days – 2 months. This brings one the reward of fasting a full year. One who does this annually is like one who fasts throughout his life.
Although voluntary in nature, the reward for this effort is tremendous. One may keep these fasts consecutively or intermittently during the month.
Among the benefits of keeping the Shawwal fasts is that they make up for deficiencies that occurred in the obligatory Ramadan fasts. It is also a sign of acceptance of one’s Ramadan fasts and a sign of commitment to continue upon good deeds.