Abu Ayyub Al-Ansari relates that Allah’s Messenger (peace be upon him) says: “Whoever fasts the month of Ramadan and then follows it with six days of fasting in the month of Shawwal, it will be as if he had fasted the year through.” (Sahih Muslim, 1163)

Thawban reported that the Prophet (peace be upon him) said: “The fast of Ramadan is like observing 10 months of fasting. Fasting six days of Shawwal is like observing two months of fasting. This together is like fasting throughout the year.” (Sahih Ibn Khuzaymah, Sunan Al-Nasai Al-Kubra; authenticated by Al-Albani)