Abu Ayyoob (may Allah be pleased with him) reported that the Messenger of Allah ﷺ said: “Whoever fasts Ramadan and follows it with six days of Shawwal, it will be as if he fasted for a lifetime”.
(Narrated by Muslim, Abu Dawood, al-Tirmidhi, al-Nisaa’i and Ibn Maajah)
The Prophet ﷺ explained this when he said: “Whoever fasts for six days after (Eid) al-Fitr has completed the year: (whoever does a good deed (hasanah) will have ten hasanah like it)”. According to another report: “Allah has made for each hasanah ten like it, so a month is like fasting ten months, and fasting six days completes the year”.
(al-Nisaa’i and Ibn Maajah. See also Saheeh al-Targheeb wa’l-Tarheeb, 1/421)
It was also narrated by Ibn Khuzaymah with the wording: “Fasting for the month of Ramadan brings the reward of ten like it, and fasting for six days brings the reward of two months, and that is the fasting of the whole year”.
The Hanbali and Shaafa’i fuqaha’ explained that fasting six days of Shawwal after fasting Ramadan makes it as if one has fasted for an entire year of obligatory fasts, because the multiplication of the reward applies even to naafil fasts, because each hasanah brings the reward of ten like it.
Another of the important benefits of fasting six days of Shawwal is that it makes up for any shortfall in a person's obligatory Ramadan fasts, because no one is free of shortcomings or sins that have a negative effect on his fasting.
On the Day of Resurrection, some of his naafil deeds will be taken to make up the shortcomings in his obligatory deeds, as the Prophet ﷺ said: “The first thing for which people will be brought to account on the Day of Resurrection will be their salah (prayer). Our Lord, may He be glorified and exalted, will say to His angels – although He knows best – ‘Look at the salah of My slave, whether it is complete or incomplete.’ If it is perfect, it will be recorded as perfect, and if something is lacking, He will say, ‘Look and see whether My slave did any voluntary (naafil) prayers.’ If he did some voluntary prayers, [Allah] will say, Complete the obligatory actions of My slave from his voluntary actions.’ Then all his actions will be dealt with in a similar manner”.
(Narrated by Abu Dawood).
And Allah knows best.