Hazrat Abu Ayyub (May Allah be pleased with him) reported: The Messenger of Allah (PBUH) said, “He who observes As-Saum (the fasts) in the month of Ramadan, and also observes As-Saum for six days in the month of Shawwal, it is as if he has observed As-Saum for the whole year.”
[Muslim Book 06, Chapter 36, Hadith # 2614]