The Prophet (Peace and blessings of Allah be upon him) commanded us to recite a lot of Tasbeeh (“Subhan-Allah”), Tahmeed (“Al-hamdu Lillaah”) and Takbeer (“Allaahu akbar”) during first ten days of Dhul Hijjah. ‘Abdullaah ibn ‘Umar (may Allah be pleased with him and his father) reported that the Prophet (Peace and blessings of Allah be upon him) said: “There are no days greater in the sight of Allah 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-Allah”), Takbeer and Tahmeed.” (Reported by Ahmad, 7/224)
Prophet Muhammad (Peace and Blessings of Allah be upon him) said: “No good deeds done on other days are superior to those done on these days [meaning the ten days of Dhul-Hijjah].”