It is Sunnah for the Muslim to fast on the first nine days of Dhu’l-Hijjah, because the Prophet (peace and blessings of Allaah be upon him) encouraged us to do righteous deeds in the first ten days of Dhu’l-Hijjah, and fasting is one of the best of deeds, which Allaah has chosen for Himself as it says in the hadeeth qudsi: “Allaah says: ‘All the deeds of the son of Adam are for him, except fasting, which is for Me and I shall reward for it.” Narrated by al-Bukhaari, 1805.