Riyad as-Salihin 1254
Abu Ayyub (May Allah be pleased with him) reported:
The Messenger of Allah (ﷺ) 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)‏‏)‏‏.‏