Abu Ayub al-Ansari relates that the Messenger of Allah (blessings and peace be upon him, his family, and companions) said, "Whoever fasts Ramadan and follows it with six days of Shawwal is as though he has fasted the entire year" [Reported by Muslim in his Sahih; Abu Dawud, Tirmidhi, and Ibn Majah; Ahmad transmitted it from Jabir, Muntaqa]. According to the opinion of many scholars, it is commendable to fast these days. Since Allah rewards each good deed with ten times its value, a Muslim who fasts six days of Shawwal after the fast of Ramadan is considered to have fasted 360 days i.e. an entire year.
Thawban (Allah be pleased with him) relates that the Messenger of Allah (blessings and peace be upon him, his family, and companions) said,
Scholars noticed that the relationship of fasting the six days of Shawwal to Ramadan is analogous to that of the voluntary prayers following obligatory prayers and that the relationship of fasting days from Sha'ban is analogous to that of the voluntary prayers preceding the obligatory prayers—they fill the imperfections that may have occurred in an obligatory action. Moreover, they indicate that the fast of Ramadan has been accepted by the will of Allah. This is because among the signs that an act of worship has been accepted, is following it up with another act of worship. Additionally, it shows that a person does not become bored of obedience and thus sets out to fast yet again after breaking his fast on the Day of 'Eid, the Day of Reward.
Based on this, the majority of the fuqaha, including the Hanafi, Shafi`i, and Hanbali school, have held that it is recommended to fast six days of the month of Shawwal. [Radd al-Muhtar; Mughni al-Muhtaj Shah al-Minhaj; Kashshaf al-Qina`]
It is permitted to fast these six days separately throughout the month, or consecutively, after Eid al-Fitr. [ibid.]
It is prohibitively disliked (makruh tahriman) and sinful to fast on the day of Eid. [Radd al-Muhtar]
The Shafi`is and Hanbalis state that it is superior to make the six fasts consecutive [Minhaj, Ghayat al-Muntaha]. Khatib al-Shirbini explains the reasoning as being, “In order to hasten to do the good, and because of the problems inevitable in delaying,” such as becoming lazy and not actually fulfilling this sunna in the end, though he points out that the sunna is fulfilled by both consecutive and non-consecutive fasting of six days in Shawwal. [Mughni al-Muhtaj]
However, it is not a condition to fast the six days of Shawwal consecutively. For instance, a person can distribute them over the month of Shawwal by fasting on Mondays and Thursdays or on the three white days i.e. the 13th, 14th and 15th of a lunar month. However, it is best to perform them immediately after the day of 'Eid.
The later Hanafi scholars differed as to which is better, fasting the six days consecutively or non-consecutively, though neither is disliked and both fulfill the sunna. [Radd al-Muhtar] However, it would be agreed that those who fear not fulfilling the recommendation due to laziness, forgetfulness or other excuses, should hasten to fast the six days consecutively immediately after Eid al-Fitr.
If possible, it is preferable to make up the missed fast days of Ramadan before fasting the six days of Shawwal.
According to Shafi'i scholars, it is possible to combine the intention of making up the missed fast days of Ramadan with the six days of Shawwal. Alternatively, one can fast the six days of Shawwal first and delay making up the missed fast days of Ramadan provided one completes them before the next Ramadan.
The time for making the intention
The intention of fasting the six days of Shawwal can be made any time before noon provided one has not done anything that voids fasting. Such is the case for all voluntary fasts, in contrast to obligatory and make up fasts for which one must make the intention.
The reward is akin to having fasted obligatory fasts the entire year days, according to Buhuti, Shirbini, and others.
Abu Hurayra (Allah be pleased with him) said that the Prophet (Allah bless him & give him peace) said,
“The strong believer is better and more beloved to Allah than the weak believer, though there is good in both. Be avid for that which benefits you. Rely on Allah and do not deem yourself incapable…”[Muslim 4816, Ibn Majah 76, Ahmad 8436]
Imam Nawawi (Allah have mercy on him) explained that the ‘strength’ in this hadith refers to, “One’s determination and ability in matters of the next life.” [Sharh Sahih Muslim]
And Allah alone gives success.