The Prophet ﷺ said, "Whoever fasts Ramadan and follows it with six days of Shawwal, it is as if he has fasted the entire year."
حدثنا أحمد بن منيع حدثنا أبو معاوية حدثنا سعد بن سعيد عن عمر بن ثابت
قال النبي ﷺ من صام رمضان ثم أتبعه ستا من شوال فذلك صيام الدهر
Sourcing. Also narrated by Muslim, Book of Fasting, Chapter on the recommendation of fasting six days of Shawwal following Ramadan, hadith 1164, from the hadith of Saad bin Saeed, with the same wording.On this topic there is also a narration from Jabir [Ahmad 3/308, 324, 344], Abu Hurairah [Bazzar (Kashf) 12/495, 496, hadith 1060], and Thawban [Ibn Majah, hadith 1715].The report of Hasan Basri is sahih, its chain...