It is narrated from Sayyiduna Thawban (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Whoever fasts Ramadan and then follows it with six days of Shawwal, that is (equal to) fasting the whole year."
Hadith Referenceالمعجم الكبير للطبراني / باب الثاء / 1438