Abu Ayyoub reported that the Messenger of Allah, salla Allahu alaihi wa sallam said

“Whoever fasts the month of Ramadhan and then follows it by fasting six days during the month of Shawwal will be rewarded as if he had fasted the entire year.”

[Muslim, at-Tirmithi, Ibn Majah, Abu Dawood and Ahmad by way of Jabir].