Riyad as-Salihin, Book 9, Hadith 1254 (In-Book) ⓘ
Abu Ayyub (May Allah be pleased with him) reported: The Messenger of Allah 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].
عن أبي أيوب رضي الله عنه أن رسول الله صلى الله عليه وسلم قال: "من صام رمضان ثم أتبعه بست من شوال كان كصيام الدهر" ((رواه مسلم)).