A companion of the Prophet ﷺ reported that the Prophet ﷺ said, "Whoever fasts Ramadan and then follows it with six days of Shawwal, it is as if he has fasted the entire year."
حدثنا النفيلي حدثنا عبد العزيز بن محمد عن صفوان بن سليم وسعد بن سعيد عن عمر بن ثابت الأنصاري
صاحب النبي ﷺ عن النبي ﷺ قال من صام رمضان ثم أتبعه بست من شوال فكأنما صام الدهر
Muslim recorded it in the chapter on siyam, in the section on the recommendation of fasting six days of Shawwal following Ramadan, hadith no. 1164, from the hadith of Saad bin Saeed, with this same chain.