Abu Ayyub Al-Ansari relates that Allah’s Messenger (peace be upon him) says: “Whoever fasts the month of Ramadan and then follows it with six days of fasting in the month of Shawwal, it will be as if he had fasted the year through.” (Sahih Muslim, 1163)
The month of Shawwal is singled out for the observance of extra fasts, since this month follows immediately after Ramadan. The six days of voluntary fasting are to the obligatory fast of Ramadan what the Sunnah prayers are to the obligatory prayers.
Thawban reported that the Prophet (peace be upon him) said: “The fast of Ramadan is like observing 10 months of fasting. Fasting six days of Shawwal is like observing two months of fasting. This together is like fasting throughout the year.” (Sahih Ibn Khuzaymah, Sunan Al-Nasai Al-Kubra; authenticated by Al-Albani)
Al-Nawawi says: “Scholars have explained that it is like observing a year of fasting because the reward of one’s good deeds are multiplied 10-fold. Therefore fasting the month of Ramadan is like fasting for 10 months and fasting six days in the month of Shawwal is like fasting for two months.” (Sahih Muslim)
Virtues of fasting six days
1. Fasting 6 days in Shawwal after Ramadan gives the person the reward of fasting throughout the year.
2. The fasts of Sha’ban and Shawwal are like the Sunnah prayers that accompany the five obligatory prayers. Like the Sunnah prayers, these extra fasts cover up for the deficiencies in our performance of our obligatory worship. On the Day of Judgment, our voluntary acts of worship will compensate for the shortcomings in how we carried out our duties. Most of us have deficiencies in our observance of our Ramadan fasts and we need something to cover up for those deficiencies.
