Abū Ayyūb (رضي الله عنه) reported that the Messenger of Allāh (ﷺ) said:
“Whoever fasts Ramadān and follows it with six days of Shawwāl, it will be as if he fasted for a lifetime.”
Muslim, Abū Dāwūd, al-Tirmidhī, al-Nasā’ī and Ibn Mājah
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:
“Allāh has made for each hasanah ten like it, so a month is like fasting ten months, and fasting six days completes the year.”
Al-Nasā’ī and Ibn Mājah
Sahīh al-Targhīb wa’l-Tarhīb, 1/421.
In numbers,
1 (month) x 10 = 10 months
6 (days) x 10 = 60 days (2 months)
10 months + 2 months = 12 months (1 year)
Imagine getting the reward for fasting a whole year and all you had to do was add 6 days to Ramadān.