On the recommendation to fast six days of Shawwal

Riyad as-Salihin (The Meadows of the Righteous)

by Imam Nawawi

228. Chapter: On the recommendation to fast six days of Shawwal

1254. Abu Ayyub reported that the Messenger of Allah, may Allah bless him and grant him peace, said, "Anyone who fasts Ramadan and then follows it with six from Shawwal, it is as he had fasted the entire year." [Muslim]