Six additional fasts
Muslims can fast another six days after Eid during the month of Shawwal, in order to keep alive the lessons learned during the month of Ramadan, and to become of those devoted to God. It is because of this that the Prophet (Allah bless him and give him peace) said: Whoever fasts of Ramadan then fasts six days in the month of Shawwal shall have the reward of having fasted the whole year. (Sahih Muslim)
Another of the important benefits of fasting six days of Shawwaal is that is 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, as the Prophet (peace and blessings of Allaah be upon him) 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.” (Narrated by Abu Dawood).
`Aa’ishah said : The Messenger of Allah, salla Allahu alaihi wa salam, used to fast Mondays and Thursdays”. [an-Nasaa’i; Sahih]
Abu Hurairah reported that the most the Prophet, salla Allahu alaihi wa salam, would fast would be Monday and Thursday. He was asked about that and he said: “The deeds of people are presented to Allah on every Monday and Thursday. Allah forgives every Muslim except for those who are deserting each other. He says: “leave them for later.” [Ahmad; Hasan]
Reward of Fasting in general (taken from Sahih Muslim):
Abu Sa’id al-Khudri (Allaah be pleased with him) reported Allaah’s Messenger (sallAllaahu alayhi wa sallam) as saying: He who observes fast for a day in the way of Allaah He would remove his face from the Hell to the extent of seventy years’ distance. No. 2572
Sahl b. Sa’d (Allaah be pleased with him) reported Allaah’s Messenger (sallAllaahu alayhi wa sallam) as saying: In Paradise there is a gate which is called Rayyan through which only the observers of fast would enter on the Day on Resurrection. None else would enter along with them. It would be proclaimed: Where are the observers of fast that they should be admitted into it? -and when the last of them would enter, it would be closed and no one would enter it. No. 2569
Abu Huraira (Allaah be pleased with him) reported Allaah’s Messenger (sallAllaahu alayhi wa sallam) as saying: Every (good) deed of the son of Adam would be multiplied, a good deed receiving a tenfold to seven hundredfold reward. Allaah, the Exalted and Majestic, has said: With the exception of fasting, for it is done for Me and I will give a reward for it, for one abandons his passion and food for My sake. There are two occasions of joy for one who fasts, joy when he breaks it, and joy when he meets his Lord, and the breath (of an observer of fast) is sweeter to Allaah than the fragrance of musk. No. 2567
Abu Huraira reported Allaah’s Messenger (sallAllaahu alayhi wa sallam) as saying: Fasting is a shield. No. 2565
….He who fasts all the year is as he who did not fast at all. The fasting of three days (a month) will be equal to the fasting of the whole year.”…. Bukhari Hadeeth No. 1862
2) Mondays and Thursdays
3) Fast of Dawood (every other days)
Narrated ‘Abdullah bin ‘Amr bin Al-‘As: Allah’s Apostle said to me, “O ‘Abdullah! Have I not been informed that you fast during the day and offer prayers all the night.” ‘Abdullah replied, “Yes, O Allah’s Apostle!” The Prophet said, “Don’t do that; fast for few days and then give it up for few days, offer prayers and also sleep at night, as your body has a right on you, and your wife has a right on you, and your guest has a right on you. And it is sufficient for you to fast three days in a month, as the reward of a good deed is multiplied ten times, so it will be like fasting throughout the year.” I insisted (on fasting) and so I was given a hard instruction. I said, “O Allah’s Apostle! I have power.” The Prophet said, “Fast like the fasting of the Prophet David and do not fast more than that.” I said, “How was the fasting of the Prophet of Allah, David?” He said, “Half of the year,” (i.e. he used to fast on every alternate day).
Afterwards when ‘Abdullah became old, he used to say, “It would have been better for me if I had accepted the permission of the Prophet (which he gave me i.e. to fast only three days a month).”
Bukhari Hadeeth No. 1858