Sahih Muslim Book 6, Hadith Number 2614.

Chapter : Excellence of observing fast for six days in the month of Shawwal after Ramadhan.

Abu Ayyub al-Ansari (Allah be pleased with him) reported Allah’s Messenger (may peace be upon him) as saying: He who observed the fast of Ramadan and then followed it with six (fasts) of Shawwal; it would be as if he fasted perpetually.