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.
Sunan an-Nasa'i Book 22, Hadith 328
← Previous Hadith "There are four things which the Prophet never gave up: Fasting 'Ashura', (fasting during) the ten days, (fasting) three days of each month, and praying two Rak'ahs before Al-Ghadah (Fajr)."

Sunan an-Nasa'i Book 22, Hadith 327 Next Hadith → "The Prophet used to fast the ten (days), and three days of each month: Monday and Thursday."

Sunan an-Nasa'i Book 22, Hadith 329