Abu Ya’la al-Mawsili relates a Hadith with an unbroken chain from Usama bin Zaid, that he said: “I used to randomly fast months during the year. The Prophet, peace upon him, said to me, ‘What about Shawwal?!’” Thereafter, as soon as Usama celebrated ‘Id al-Fitr, he began fasting Shawwal until he reached the end of the month.
Fasting Shawwal is like fasting Sha’ban. Each of these two months is wedded to Ramadan, one precedes it and one follows it. We have previously mentioned, in the context of discussing the virtue of fasting Sha’ban, that keeping fast during it is better than fasting the sacred months –there is a slight difference of opinion around this issue. 
Fasting Ramadan and then following that with six days from Shawwal is like fast perpetually, because good deeds are multiplied [minimally] tens times. This idea has been explicitly mentioned in the Hadith of Thawban in which he related from the Prophet, peace upon him, “Fasting Ramadan is like fasting ten months, and fasting six days is like fasting two months. [Together] it is fasting an entire year.” This means fasting Ramadan and then the six days after it. This Hadith is related by Imam Ahmad, al-Nasa’i, with this wording. It is also related by Ibn Hibban in his Sahih. Abu Hatim al-Razi declared it to be a sound narration.  Imam Ahmad mentions that there is no sounder Hadith relating to this issue. He declined to rule on its probity in another narration.
There is no difference in this regard if Ramadan is thirty or twenty-nine days. This is based on the Hadith of the Prophet, peace upon him, “The two months of ‘Id, Ramadan and Dhu’l Hijjah, will never be lessened.”  The narrator of this Hadith said that this means they will never be lessened in their ruling, regardless if they are thirty or twenty-nine days in duration. So if Ramadan [regardless of its length] is followed by six days of Shawwal, it is like fasting throughout the year in any case. For this reason, Ishaq bin Rahawayhi disliked referring to Ramadan as lessened, even if it was twenty-nine days.
If someone were to say, “If anyone fasts six days of the year at any time, they would attain this virtue. Why is it limited to Shawwal?” The response would be, “Fasting them in Shawwal is connected to Ramadan in terms of their virtue. Hence, one fasting them necessarily gains the reward of fasting perpetually.” This exclusivity for Shawwal has been mentioned by ‘Abdullah bin al-Mubarak, as has been related by Imam al-Tirmidhi in his Jami’. Perhaps ibn al-Mubabak was insinuating a Hadith related from Umm Salama in which she mentioned, “Whoever fasts the day after ‘Id al-Fitr, it is as if they have fasted [a day of] Ramadan.”
Fasting immediately after Ramadan has many benefits. Among them:
1. Fasting six days after Ramadan brings the reward of fasting perpetually as just discussed.
2. Fasting Sha’ban and Shawwal is like praying the supererogatory prayers that precede and follow the obligatory prayers. [Like those prayers,] this fast compensates for the defects and flaws existing in the obligatory act. The obligatory acts will be rendered whole by the supererogatory acts on the Day of Resurrection. This meaning has been narrated from the Prophet from many different paths. Most people have defects and flaws in their obligatory fast, and are in need of something to mend and complete it.
For this reason, the Prophet, peace upon him, used to dislike someone saying, “I fasted all of Ramadan, or I stood in prayer at night the entirety of Ramadan.” Abu Bakrah said, “I do not know if he [the Prophet, peace upon him], was discouraging boastfulness or if he was implying that the fast is inevitably defective.” 
‘Umar bin ‘Abd al-‘Aziz, may Allah have mercy on him, used to say, “Whoever finds nothing to give charity with let him fast.” This means that one who finds nothing to spend for the Festival day charity (Sadaqa al-Fitr) at the end of Ramadan, let him fast the day after ‘Id-al-Fitr, because it takes the place of that charity in expiating for the defectiveness of the fast, just as it takes the place of charity as an expiation for violated oaths, and other acts, such as accidental killing, sexual relations during the day in Ramadan, or declaring ones wife untouchable (Dhihar).
(to be continued)
 Some of the manuscripts that this publication is based on mention that “there is no difference of opinion around this issue,” while others mention “there is a slight difference of opinion around this issue.” I have chosen the latter as being more accurate.
 Ahmad, al-Musnad, 5:280; Ibn Hibban, 5:258
 Bukhari, 4:124; Muslim, 1089; Abu Dawud, 2323
 Jami’ Usul, 11:735
Assalaamu Alaikum Shaikh. Jazakhallah Khair for the information. I don't know if you mind answering questions in this manner or if you'd prefer a different method. I have two questions related to the fast of Shawwal. 1. Is it necessary to fast the first six days immediately following Ramadan? Or can we fast any six days within Shawwal. 2. If any of the six days of Shawwal fall on a Monday or Thursday, will we get the blessings of fasting on those days as well? Or will we only get the reward for one or the other? Or is it based on our intention? May Allah reward you for your service to Islam. Ameen.
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