I heard the Messenger of Allah ﷺ say, "Whoever fasts Ramadan and follows it with six days of Shawwal, it is as though he fasted the entire year."
حدثنا عبد الله بن يزيد حدثنا سعيد يعني ابن أبي أيوب حدثني عمرو بن جابر الحضرمي قال سمعت
سمعت رسول الله ﷺ يقول من صام رمضان وستا من شوال فكأنما صام السنة كلها
Its isnad is daif because of Amr bin Jabir Hadrami, and likewise Haythami declared it daif in 3/183 for the same reason. However, the hadith is sahih, reported by Muslim with its wording in 2/822, number 1164, and its repetitions in the chapter on fasting, on the recommendation of fasting six days of Shawwal, from Abu Ayub. Likewise Abu Dawud 2/336, number 2433, with similar wording, and likewise Tirmidhi 3/123, number 759, who said, hasan sahih, and likewise Ibn Majah 1/547, number 1716, and Ibn Abu Shayba 3/97, and Tabarani in Al-Kabir 4/134, number 3902, and Ibn Hibban 928 in Al-Mawarid, from Thawban.