The Fast of Dhu al-Hijjah

The Prophet would fast the first nine days of the month Dhu al-Hijjah and embrace the tenth day, the Eid al-Adha, in this state of worship. He would emphasize the value and virtue of fasting on the day of Arafah, the day before Eid al-Adha, and that it expiates for the sins of a whole year. (Muslim, Sawm, 197) So it is mustahab to fast on the first nine days of Dhu al-Hijjah.