Yahya bin Sam يحيى بن سام · The Book of Fasting

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."

"The Messenger of Allah used to enjoin fasting three days: The first Thursday, and Monday and Monday."