The Messenger of Allah ﷺ said, "Whoever fasts Ramadan and follows it with six days of Shawwal, it is as if he has fasted the entire year."
أن رسول الله ﷺ قال: من صام رمضان وأتبعه ستا من شوال، كان كصيام الدهر.
Muslim narrated it in the Book of Fasting (1164) through several routes from Saad bin Saeed, the brother of Yahya bin Saeed, from Umar bin Thabit bin Harith Khazraji, from Abu Ayub Ansari, and he related it.