Fasting six days of Shawwaal after the obligatory fast of Ramadaan is Sunnah Mustahabbah. It is recommended for the Muslim to fast six days of Shawwaal, and in this there is great virtue and an immense reward.
Whoever fasts these six days will have recorded for him a reward as if he had fasted a whole year.
Abu Ayyoob radi Allaahu 3Anhu reported that the Messenger of Allaah SallAllaahu 3Alayhi wa Sallam said: “Whoever fasts Ramadaan and follows it with six days of Shawwaal, it will be as if he fasted for a lifetime (whole year).” (1)
The Prophet SallAllaahu 3Alayhi wa Sallam explained this when he said: “Whoever fasts for six days after (3Eed) al-Fitr has completed the year: (whoever does a good deed (hasanah) will have ten hasanah like it).”
According to another report: “Allaah has made for each hasanah ten like it, so a month is like fasting ten months, and fasting six days completes the year.” (2)
It was also narrated by Ibn Khuzaymah with the wording: “Fasting for the month of Ramadaan brings the reward of ten like it, and fasting for six days brings the reward of two months, and that is the fasting of the whole year.”
Another of the important benefits of fasting six days of Shawwaal is that it makes up for any shortfall in a person’s obligatory Ramadaan fasts, because no one is free of shortcomings or sins that have a negative effect on his fasting. On the Day of Resurrection, some of his naafil deeds will be taken to make up the shortcomings in his obligatory deeds.
The Prophet SallAllaahu 3Alayhi wa Sallam said: “The first thing for which people will be brought to account on the Day of Resurrection will be their salaah (prayer). Our Lord, may He be glorified and exalted, will say to His angels – although He knows best – ‘Look at the salaah of My slave, whether it is complete or incomplete.’ If it is perfect, it will be recorded as perfect, and if something is lacking, He will say, ‘Look and see whether My slave did any voluntary (naafil) prayers.’ If he did some voluntary prayers, [Allaah] will say, Complete the obligatory actions of My slave from his voluntary actions.’ Then all his actions will be dealt with in a similar manner.” (3)
Rulings on Fasting in Shawwaal for Women (Updated) (must read)References: (1) Muslim, Abu Dawood, At-Tirmidhee, an-Nisaa’ee and Ibn Maajah (2) An-Nisaa’ee and Ibn Maajah. Saheeh al-Targheeb wa’l-Tarheeb, 1/421. (3) Narrated by Abu Dawood http://islamqa.info/en/ref/7859