Fiqh-us-Sunnah Volume 2, Supererogatory Prayer, Fiqh 2.154.

Ibn ‘Abbas reports that the Prophet sallallahu alehi wasallam said: “No good deeds done on other days are superior to those done on these days [meaning the ten days of Zhul-Hijjah].” The companions asked: “O Messenger of Allah, not even jihad in the way of Allah?” He said: “Not even jihad, save for the man who puts his life and wealth in danger [for Allah’s sake] and returns with neither of them.” This is related by the group save Muslim and an-Nasa’i.

Ahmad and at-Tabarani record from Ibn ‘Umar that the Messenger of Allah said: “There is no day more honourable in Allah’s sight and no acts more beloved therein to Allah than those in these ten days. So say tahlil [“There is no God but Allah”], takbir [Allah is the greatest] and tahmid [“All praise is due to Allah”] a lot [on those days].”

Ibn ‘Abbas says about the ‘ayah, “Remember Allah during the well known days,” that it refers to the first ten days of Zhul-Hijjah. This is related by al-Bukhari. Sa’id ibn Jubair would push himself very hard [to do good deeds] during these ten days.

Al-Auza’i says: “It has reached me that a deed on one of the ten days is similar to fighting in the way of Allah, fasting during its days and guarding during its nights, except for him who becomes a martyr.” As to its source, he adds: “A man from the tribe of Bani Makhzum related that to me from the Prophet.”

Abu Hurairah relates that the Prophet said: “There are no days more loved to Allah for you to worship Him therein than the ten days of ZhulHijja. Fasting any day during it is equivalent to fasting one year and to offer salatul tahajjud (late-night prayer) during one of its nights is like performing the late night prayer on the night of power. [i.e., Lailatul Qadr].” This is related by at-Tirmizhi, Ibn Majah, and al-Baihaqi.