In the name of Allah, the Gracious, the Merciful
It is highly recommended to fast six days of the month of Shawwal after completing the fast of Ramadan. Fasting for these six days in addition to Ramadan is like fasting for a year (al-dahr) and one who keeps up this hadith will be rewarded as if they fasted every day in their life.
Abu Ayyub reported: The Messenger of Allah, peace and blessings be upon him, said:
مَنْ صَامَ رَمَضَانَ ثُمَّ أَتْبَعَهُ سِتًّا مِنْ شَوَّالٍ كَانَ كَصِيَامِ الدَّهْرِ
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 fasted for the entire year.
Source: Ṣaḥīḥ Muslim 1164, Grade: Sahih
The reason it will be as if he or she has fasted the whole year is because Allah multiplies sincere good deeds by ten or more. Al-Nawawi comments on this tradition, writing:
قَالَ الْعُلَمَاءُ وَإِنَّمَا كَانَ ذَلِكَ كَصِيَامِ الدَّهْرِ لِأَنَّ الْحَسَنَةَ بِعَشْرِ أَمْثَالِهَا فَرَمَضَانُ بِعَشَرَةِ أَشْهُرٍ وَالسِّتَّةُ بِشَهْرَيْنِ
The scholars said that it is like fasting for a year, since good deeds are multiplied by ten like it. Thus, Ramadan is like ten months and six of Shawwal is like two months.
Source: Sharḥ al-Nawawī ‘alá Ṣaḥīḥ Muslim 8/56
As such, thirty days of Ramadan is equal to three hundred days of fasting and six days of Shawwal is equal to sixty days of fasting, for a total of three hundred and sixty days or about one year. This understanding is confirmed by another reliable narration.
Thawban reported: The Messenger of Allah, peace and blessings be upon him, said:
صِيَامُ رَمَضَانَ بِعَشَرَةِ أَشْهُرٍ وَصِيَامُ السِّتَّةِ أَيَّامٍ بِشَهْرَيْنِ فَذَلِكَ صِيَامُ السَّنَةِ
Fasting the month of Ramadan is like ten months of fasting and fasting six days of Shawwal is like two months of fasting. Thus, they are like fasting for a year.
Source: Ṣaḥīḥ Ibn Khuzaymah 2115, Grade: Sahih
Therefore, we should take advantage of this opportunity to gain a tremendous amount of rewards for a comparatively small amount of effort. We already know the tremendous reward of fasting Ramadan and voluntary Sunan days, so imagine receiving the reward as if you fasted your entire lifetime.
However, if one finds it difficult to fast six days of Shawwal, just keeping up a habit of fasting three days a month in addition to Ramadan will earn the same reward as fasting every day.
Abu Qatadah reported: The Prophet, peace and blessings be upon him, said:
صَوْمُ ثَلاَثَةٍ مِنْ كُلِّ شَهْرٍ وَرَمَضَانَ إِلَى رَمَضَانَ صَوْمُ الدَّهْرِ
Fasting three days of every month and Ramadan after Ramadan is like fasting for a lifetime.
Source: Ṣaḥīḥ Muslim 1162, Grade: Sahih
One of the purposes of fasting in Shawwal is to continue the good habit of fasting that we have developed during Ramadan. It is important to capitalize on the positive momentum we built up in the last month. If we do not fast any days of Shawwal, it will be much more difficult to make fasting a regular habit throughout the year.
Voluntary Sunan days of fasting we should work into our routines are fasting on the days of Arafat and Ashura, fasting Mondays and Thursdays, and fasting three days in the middle of every month.
Abu Qatadah reported: A man came to the Messenger of Allah, peace and blessings be upon him, and he said, “What is your view about fasting on the day of Arafat?” The Prophet said:
يُكَفِّرُ السَّنَةَ الْمَاضِيَةَ وَالْبَاقِيَةَ
It will expiate the sins of the previous and upcoming years.
The man said, “O Messenger of Allah, what is your view about fasting the day of Ashura?” The Prophet said:
يُكَفِّرُ السَّنَةَ الْمَاضِيَةَ
It will expiate the sins of the year.
Source: Ṣaḥīḥ Muslim 1162, Grade: Sahih
Abu Huraira reported: The Messenger of Allah, peace and blessings be upon him, said:
تُعْرَضُ الْأَعْمَالُ يَوْمَ الِاثْنَيْنِ وَالْخَمِيسِ فَأُحِبُّ أَنْ يُعْرَضَ عَمَلِي وَأَنَا صَائِمٌ
The deeds are presented on Monday and Thursday. Thus, I love for my deeds to be presented while I am fasting.
Source: Sunan al-Tirmidhī 747, Grade: Sahih
Abu Dharr reported: The Messenger of Allah, peace and blessings be upon him, said:
إِذَا صُمْتَ مِنْ الشَّهْرِ ثَلَاثَةَ أَيَّامٍ فَصُمْ ثَلَاثَ عَشْرَةَ وَأَرْبَعَ عَشْرَةَ وَخَمْسَ عَشْرَةَ
O Abu Dharr, if you are going to fast three days of the month, then fast on the thirteenth, fourteenth, and fifteenth days.
Source: Sunan al-Tirmidhī 761, Grade: Sahih
The consistency of a good practice is more important than its perceived size, as Allah loves good deeds that are done regularly even if they seem minor. So even coming out of Ramadan with a new good habit of fasting three days a month is a decisive spiritual victory.
Aisha reported: The Messenger of Allah, peace and blessings be upon him, said:
وَاعْلَمُوا أَنَّ أَحَبَّ الْعَمَلِ إِلَى اللَّهِ أَدْوَمُهُ وَإِنْ قَلَّ
Know that the most beloved deed to Allah is that which is done regularly, even if it is small.
Source: Ṣaḥīḥ al-Bukhārī 6099, Grade: Sahih
To conclude, let us reinforce the good habits we have cultivated during Ramadan and make fasting for some days a regular habit in every month, even if only a few days.
Success comes from Allah, and Allah knows best.