Abu Ayyub (may Allah be pleased with him) reported that the Messenger of Allah (peace and blessings of Allah be upon him) 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 Dawud, al-Tirmidhi, al-Nasai and Ibn Majah).