It is as if he has observed the Fasting for the whole year

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)

Abu Ayyub (رضي الله عنه) reported:

The Messenger of Allah (صلى الله عليه وسلم ) 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.”
[Sahih Muslim]

Source: From Riyad-us-Saliheen (Book 9, Hadith 1254 ) : Dar-us-salam English Publication