Hafsah relates: "There are four things that the Prophet never abandoned: Fasting the day of 'Aashooraa', fasting the first ten days of Thul-Hijjah, fasting three days of every month and praying the two Raka's before the dawn prayer." [Ahmad and An-Nasaa'ee] Note that "the first ten" means the first nine days since the tenth will be the 'Eed (i.e. the day of sacrifice). This narration is classified by Ibn Hibbaan as authentic but some other scholars classified it as a weak narration. However, there are other narrations by Al-Bukhari and Muslim which prove the legality of every section of the above narration, like the fasting of the 13th, 14th, and 15th of every month, and the two Raka's before the dawn prayer. There are also authentic narrations about urging people to perform good deeds on the ten days of Thul-Hijjah and there is no doubt that fasting is the best act of worship, so how could a person refrain from getting this great benefit and reward?

For more benefit, please refer to Fatwa 85045.

Allaah knows best.