Sayyiduna Abu Ayyub al-Ansari (may Allah be pleased with him) narrates: I heard the Messenger of Allah (peace and blessings be upon him) saying: "Whoever fasts Ramadan and then follows it with six (days) of Shawwal, that is the fasting of the whole year."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8431