Regarding the the superiority of the first ten days of Dhul-Hijjah, the Messenger of Allah صلى الله عليه وسلم said:

“There are no other days that are as great as these in the sight of Allah, the Most Sublime. Nor are there any deeds more beloved to Allah then those that are done in these ten days. So increase in tahlîl (to say lâ illaha illallah), takbîr (to say Allahu akbar) and tahmîd (to say alhumdulillâh).”

[Reported by at-Tabarânî in al-Mu’jum al-Kabîr]