It was narrated from from Abu Ayyub al-Ansari (Allah be pleased with him) that the Messenger of Allah (peace be upon him) said: “Whoever fasts Ramadan then follows it with six days (fasting) of Shawwal, it is as if he fasted a lifetime.”
عن أبي أيوب الأنصاري رضي الله عنه أنه حدثه أن رسول الله صلی الله عليه وسلم قال من صام رمضان ثم أتبعه ستا من شوال کان کصيام الدهر
[Sahih Muslim, Book of Fasting, Hadith: 2758]