It is narrated from Sayyiduna Thawban (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: “(The fasting of a year is like this:) the fasting of one month is equal to ten months, and after that, the fasting of six days is equal to two months, thus making twelve months and completing a whole year.” That is, one month of Ramadan and after it, six fasts of Shawwal.
Hadith Takhrijاس روایت کی سند صحیح ہے۔ دیکھئے: [ابن ماجه 1715] ، [ابن حبان 3635] ، [موارد الظمآن 928]
Brief Explanation
(Commentary on Hadiths 1791 to 1793)
It is stated in Ibn Majah: This Divine statement: «﴿مَنْجَاءَبِالْحَسَنَةِفَلَهُعَشْرُأَمْثَالِهَا ...﴾ [الانعام : 160]» is in accordance with the fact that whoever brings a good deed will receive the reward of ten good deeds. Now, multiply 36 by 10, and it becomes 360 days. Therefore, whoever fasted for 36 days received the reward of fasting for the entire year. How great is the grace and kindness of the Lord of Majesty, the Most Merciful and Generous, that the fasting is for just over a month, but the reward is for the whole year. These fasts of Shawwal can be observed at the beginning, middle, or end of Shawwal, and they can be kept consecutively or separately. However, from «ثُمَّأَتْبَعَهُسِتَّةً», it appears that it is better to observe them consecutively at the beginning of Shawwal.
And Allah knows best.
It should be noted that some Imams have considered the six fasts of Eid after Ramadan to be disliked (makruh), which is not correct. It is possible that they were unaware of the aforementioned authentic ahadith.
Allamah Waheed al-Zaman rahimahullah writes: And in front of the saying of the Messenger of Allah sallallahu alayhi wa sallam, no one else's statement is to be listened to, and to light a lamp in front of the sun is foolishness.