In the Hadīth of Ibn ‘Abbās (r) that the Messenger of Allāh saws said in Sahih Muslim:
“When you see the new moon of Dhu’l-Hijjah, if one of you wants to offer a sacrifice, let him refrain from (cutting) his hair and nails.”

And He saws said:
“There are no days in which good deeds are more beloved to Allah than these days.”

Included in the general wording of the statement: ‘good deeds’ is: fasting; fasting is among the best of deeds, and Allāh has singled out for Himself (to reward). So fasting all nine days is a Sunnah, as the general statement of the Hadīth indicates, and as is the view of the majority of scholars. … More RULINGS ON THE FIRST 10 DAYS OF DHUL-HIJJAH – WHAT DO I DO ON EID AL ADHA – TAKBEERAT – SACRIFICE – NAILS & HAIR – IS SHAVING MY BEARD HARAM