Abu Aiyub Al-Ansari (RAA) narrated that The Messenger of Allah (ﷺ) said:
"Whoever fasts during the month of Ramadan and then follows it with six days of Shawwal will be (rewarded) as if he had fasted the entire year." Related by Muslim.
English reference: Book 5, Hadith 701
Arabic reference: Book 5, Hadith 681