Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) said: "Whoever fasts Ramadan and then after it also fasts six days of Shawwal, it is as if he fasted the whole year."
Al-Bazzar recorded it. It has various chains, and the narrators of some of them are the narrators of the Sahih.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصيام / 5099