"I heard the Messenger of Allah say: "Whoever fasts three days of each month has fasted the month in full' or 'will have (the reward of) fasting the month."' 'Asim was in doubt.
Sunan an-Nasa'i Book 22, Hadith 321
← Previous 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 Next Hadith → "I heard the Messenger of Allah say: 'It is a good fast to fast three days of each month".

Sunan an-Nasa'i Book 22, Hadith 322