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 ﷺ قال: "من صام رمضان ثم أتبعه بست من شوال كان كصيام الدهر" ((رواه مسلم)).