Narrated Ibn `Abbas: The Prophet (ﷺ) said, "No good deeds done on other days are superior to those done on these (first ten days of Dhul Hijja)." Then some companions of the Prophet (ﷺ) said, "Not even Jihad?" He replied, "Not even Jihad, except that of a man who does it by putting himself and his property in danger (for Allah's sake) and does not return with any of those things." (Sahih al-Bukhari 969)
The Prophet (peace and blessings of Allaah be upon him) commanded us to recite a lot of Tasbeeh ("Subhan-Allaah"), Tahmeed ("Al-hamdu Lillaah") and Takbeer ("Allaahu akbar") during this time. ‘Abdullaah ibn ‘Umar (may Allaah be pleased with him and his father) reported that the Prophet (peace and blessings of Allaah be upon him) said: "There are no days greater in the sight of Allaah and in which righteous deeds are more beloved to Him than these ten days, so during this time recite a great deal of Tahleel ("La ilaaha ill-Allaah"), Takbeer and Tahmeed."
Musnad Ahmad 7/224; Musnad Ahmad 5446