It is narrated from Sayyiduna Abu Ayyub (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 fasts of Shawwal, it is as though he fasted perpetually." Hafs said: Then I met Sa'd, and he (too) narrated this hadith to me.
Hadith Referenceالمعجم الكبير للطبراني / باب الخاء / 3897