" Whoever fasts Ramadan and follows it with six days of Shawwal, it will be as if he fasted for a lifetime. [Muslim and others]
Scholars explain that fasting six days of Shawwal after fasting Ramadan is as if one has fasted for an entire year of obligatory fasts and also it makes up for any shortfall in a persons obligatory Ramadan fasts, because no one is free of shortcomings or sins that have a negate effect on his fasting.