Abu Ayyub al-Ansari told that God’s messenger said, “If anyone fasts during Ramadan, then follows it with six days in Shawwal, it will be like a perpetual fast.” Muslim transmitted it.
The Virtue of Six Days in Shawwal
From the perspective of classical scholarship, this noble hadith from Sahih Muslim contains profound wisdom regarding voluntary fasting. The Prophet's description of "perpetual fast" (ka's-sana bid-dahri) indicates that whoever observes Ramadan and follows it with six days of Shawwal receives the reward of fasting an entire year.
Scholarly Explanation of the Reward
Imam an-Nawawi explains in his commentary on Sahih Muslim that each good deed is rewarded tenfold, thus Ramadan equals ten months and six days equal sixty days (two months), completing the year's reward. This demonstrates Allah's boundless generosity.
The scholars emphasize that these six days need not be consecutive, though it is preferable to observe them immediately after Eid to maintain spiritual momentum. The fasting may be done any time during Shawwal, provided one has completed Ramadan first.
Juridical Rulings and Conditions
According to the Hanafi, Shafi'i, and Hanbali schools, this practice is highly recommended (mustahabb). One must have observed the complete Ramadan fast; if any days were missed, they must be made up before observing the Shawwal fasts to attain this special reward.
The wisdom behind this practice is to train the soul in continuous obedience and to compensate for any deficiencies in the obligatory fast of Ramadan, as the voluntary deeds complete what is lacking in obligatory acts.