" I heard the Messenger of Allah say: 'The month of patience ( Ramadan) and three days of each month is fasting for a lifetime."' (Sahih).
Sunan an-Nasa'i Book 22, Hadith 319
← Previous Hadith "The Messenger of Allah commanded me to sleep after praying Witr, to perform Ghusl on Friday and to fast three days of each month." (Sahih).

Sunan an-Nasa'i Book 22, Hadith 318 Next Hadith → "The Messenger of Allah said: 'Whoever fasts for three days of each month, he has fasted for a whole lifetime.' Then he said: Allah has spoken the truth in His book: Whoever brings a good deed shall have ten times the like thereof to his credit.

Sunan an-Nasa'i Book 22, Hadith 320