Ibn ‘Abbas (may Allah be pleased with him and his father) narrated that the Prophet (may Allah’s salah and salam be upon him) said, "There are no days during which righteous deeds are more beloved to Allah than these days – meaning the ten days of Dhul-Hijjah.” The Companions present asked, “Not even jihad (struggling in Allah’s path to uphold His religion)?” He replied, “Not even jihad, except in the case of a man who risks his life and property in order to overcome the enemy and does not return with either of those things.”

Similarly, Ahmad reported from Ibn ‘Umar (may Allah be pleased with him and his father) that the Prophet (may Allah’s salah and salam be upon him) said, “There are no days greater in virtue nor more beloved to Allah for righteous deeds to be performed in than these ten days. Thus, you should increase in Tahlil (to say La Ilaha illAllah), Takbir (to say Allahu Akbar), and Tahmid (to say Alhamdulillah) in these days.”

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The Virtues of these Ten Days and the Reasons for their Superiority over Other Days:

1. Allah, the Most High, took an oath by these ten days when He stated, ((I swear by the time of fajr, and I swear by the first ten nights.)) [89:1-2]. Ibn Kathir commented, What is intended by ((...the first ten nights)) is the (days of) the (first ten nights of Dhul-Hijjah as stated by Ibn ‘Abbas.

2. These are appointed days as in the statement of Allah, the Most High, ((and mention the name of Allah on the appointed days.)) [22:28]. Ibn Kathir commented, “Ibn ‘Abbas said: ‘The appointed days are the first ten days of Dhul-Hijjah.’"

3. Allah, the Most High, loves for righteous actions to be performed in them and they are the most virtuous days to Him – the Most Majestic and Most High.

4. The coinciding of a noble time for all Muslims and a noble place specific for those in Makkah such as Hajj pilgrims and others.

5. The coinciding of the greatest acts of worship in these days. Ibn Hajar said, “What is apparent for the distinction of the first ten days of Dhul-Hijjah, is the gathering of the greatest acts of worship in them, which are: the prayer, fasting, charity, and Hajj. Such does not occur in other days.”

6. Among these days is the Day of ‘Arafah which is “the Witnessed Day” in which Allah completed the religion of Islam and fasting this day expiates the sins of two years.

7. Among these days is the Day of Sacrifice, which is, without exception, the greatest day of the year. It is the greatest day of Hajj, comprising various acts of obedience that do not come together on any other day.

8. The prescribed act of slaughtering during these days commences on the tenth of Dhul-Hijjah.
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Among the strongly recommended actions to be performed in these days are to increase in Takbir (to say Allahu Akbar), Tahlil (to say La Ilaha illAllah), and Tahmid (to say Alhamdulillah).
In support of this, Allah, the Most High, stated, ((They will come so that they may witness things of benefit to them and mention the name of Allah on appointed days over the beasts of cattle which He provided for them.)) [22:28]. Also what supports this is the aforementioned Hadith, “Thus, you should increase in Takbir (to say Allahu Akbar), Tahlil (to say La Ilaha illAllah), and Tahmid (to say Alhamdulillah) in these days.”

Likewise, al-Bukhari reported that Ibn ‘Abbas said, “Ibn 'Umar and Abu Hurayrah would go out to the market places during these ten days and utter the Takbir (to say Allahu Akbar) and people would consequently utter the Takbir also.” The meaning is that people would remember to utter the Takbir due to the Takbir of Ibn ‘Umar and Abu Hurayrah thus everyone would utter the Takbir individually. As for congregational Takbir, then this is not legislated.