It is narrated from Sayyiduna Abu Ayyub al-Ansari (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever fasts Ramadan and then follows it with six fasts of Shawwal, it is as though he fasted perpetually."
Hadith Referenceالمعجم الكبير للطبراني / باب الخاء / 3896