It is narrated from Sayyiduna Abu Ayyub (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever fasts Ramadan and six days of Shawwal, it is as though he fasted the whole year."
Hadith Referenceالمعجم الكبير للطبراني / باب الخاء / 3889