“Whoever fasts Ramadān and then follows it with six days from Shawwāl, it will be as if he has fasted for a lifetime.”

Reported by Muslim (no. 1164), from the narration of Abū Ayyūb al-Anṣārī — may Allah be pleased with him.

Brief Explanation of the Hadith

It was from the guidance of the Prophet — may Allah’s peace and blessings be upon him — to be consistent in righteous deeds and to encourage them. Among these is fasting six days of Shawwāl: whoever fasts Ramadān and then fasts six days of Shawwāl—whether consecutive or separate, at the beginning, middle, or end of the month—will have the reward of fasting an entire year, for each good deed is multiplied tenfold: Ramadān equals ten months, and the six days equal two months. As Allah, the Exalted, said: {Whoever comes with a good deed will have ten times the like thereof} al-Anʿām: 160.
Whoever consistently fasts them every year will be as one who has fasted for his entire lifetime.
Shawwāl was singled out for this virtue because the soul tends, after Ramadān, toward eating and drinking, making fasting in it more difficult—and thus its reward greater.
It is best for one who owes missed days from Ramadān to make them up first, then fast the six days—so as to avoid scholarly disagreement. However, if he fasts the six before making up the missed days, there is no harm according to the majority of scholars.
The hadith indicates the recommendation of fasting six days of Shawwāl, clarifies their virtue, and highlights Allah’s grace and generosity in multiplying good deeds for His servants.