Fasting six days of Shawwal

 
"مَنْ صَامَ رَمَضَانَ ثُمَّ أَتَبَعَهُ بِسِت مِنْ شَوَال فَكَأَنَّمَا صَامَ الدَّهْرَ"

 Rasulullah (Sallallahu alayhi wasallam) has said: Whoever fasts in Ramadhan and thereafter follows it with six (days) from Shawwal, it will be as though he had fasted the (entire) year.

(Abu Dawud, Muslim)