The Messenger of Allah 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 أبو معاوية حدثنا سعد بن سعيد عن عمر بن ثابت
قال رسول الله ﷺ من صام رمضان ثم أتبعه ستا من شوال فذلك صيام الدهر
Its isnad is sahih. Saad bin Saeed is Ibn Amr Ansari Madani, trustworthy. His hadith is found in Muslim and the four (Sunan compilers). The same applies to Umar bin Thabit, and both of them are from the Followers. The hadith was mentioned previously at 14645 and 14236.