"The Messenger of Allah used to command us to fast the three days with the shining bright nights (Al-Ayam Al-Bid), the thirteenth, fourteenth and fifteenth."
Sunan an-Nasa'i Book 22, Hadith 343
← Previous Hadith the Prophet commanded them to fast the three days of Al-Bid. He said: "That is (equivalent to) fasting for the whole month."

Sunan an-Nasa'i Book 22, Hadith 342 Next Hadith → "I asked the Messenger of Allah about fasting and he said: 'Fast one day of the month.' I said: 'Fast one day of the month.' I said: 'O Messenger of Allah, let me do more, let me do more.' He said: 'Let me do more, let mo do more; I am able for it.' Then the Messenger of Allah fell silent until I thought that he was going to refuse my request. Then he said: 'fast three days of each month."'

Sunan an-Nasa'i Book 22, Hadith 344