The Book of Fasting
كتاب الصيام
Chapter 83: How to Fast Three Days of Each Month, And Mentioning The Differences Reported by The Narrators In The Narration Regarding that
the Messenger of Allah used to fast nine days of Dhul-Hijjah, the day of 'Ashura', and three days of each month: The first Monday of the month, and two Thursday.
"The Prophet used to fast the ten (days), and three days of each month: Monday and Thursday."
Chapter 84: Mentioning The Differences Reported From Musa Bin Talhah In The Narration About Fasting three days of each month
That the Prophet commanded a man to fast on the thirteenth, fourteenth and fifteenth.
"Ubayy said: 'A Bedouin came to the Messenger of Allah, and he had a rabbit that he had grilled and some bread. He placed it before the Prophet then he said: "I found it bleeding." The Messenger of Allah said to his Companions: "It doesn't matter; eat." And he said to the Bedouin: "Eat," He said: "I am fasting." He said: "If you want to fast then you should fast the shining days of Al-Bid: The thirteenth, fourteenth and fifteenth."
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."
the Prophet commanded them to fast the three days of Al-Bid. He said: "That is (equivalent to) fasting for the whole month."