the Messenger of Allah used to enjoin (fasting) these days of Al-Bid and he said: "That is (equivalent to) fasting for the whole month."
Sunan an-Nasa'i Book 22, Hadith 341
← Previous Hadith "A rabbit that a man had grilled was brought to the Prophet and when he offered it to him he said: 'O Messenger of Allah, I saw some blood on it." The Messenger of Allah did not eat it, but he said to those who were with him: "Eat; if I felt like it, I would have eaten it.' There was a man sitting, and the Messenger of Allah said: 'Come and eat with the people.' He said: 'O Messenger of Allah, I am fasting.' He said: 'Why don't you fast Al-Bid?' He said: 'What are they?' He said: "The thirteenth, fourteenth and fifteenth?"'

Sunan an-Nasa'i Book 22, Hadith 340 Next 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