It is authentically reported that

Q: We would like your Eminence to enlighten us regarding the ruling on reciting Takbir (saying: "Allahu Akbar [Allah is the Greatest]") on the days of Tashriq (11th, 12th and 13th of Dhul-Hijjah) and the days of `Eid-ul-Fitr (the Festival of Breaking the Fast) in groups. This happens when the Imam (the one who leads congregational Prayer) says after each Salah (Prayer): Allahu Akbar, Allahu Akbar, Allahu Akbar, La Ilaha Illa Allah, Allahu Akbar, Allahu Akbar Wali-l-Lahi-l-Hamd i.e. Allah is the Greatest (thrice), there is no god but Allah, Allah is the Greatest, Allah is the Greatest and all Praise be to Allah. Then, the congregation repeats the same three times in one loud and melodious voice. They do so after each Salah for three days and this habit is prevalent in some of the southern towns.

A: Reciting Takbir is recommended at any time during the two nights before `Eid-ul-Fitr (the Festival of Breaking the Fast) and `Eid-ul-Adha (the Festival of the Sacrifice) and during the first ten days of Dhul-Hijjah. It is also recommended after each obligatory Salah starting from the Fajr (Dawn) Prayer of the day of `Arafah (9th of Dhul-Hijjah) until the end of the days of Tashriq (11th, 12th and 13th of Dhul-Hijjah) for Allah (Exalted be He) says: (He wants that you) must complete the same number (of days), and that you must magnify Allâh [i.e. to say Takbîr (Allâhu Akbar; Allâh is the Most Great)] for having guided you He (Exalted be He) also says: And remember Allâh during the appointed Days. Moreover, it is narrated that Imam Ahmad (may Allah be merciful to him) was asked: "What is the ruling on reading Takbir from Fajr Prayer on the day of `Arafah until the last day of Tashriq? Imam Ahmad said: "This is recommended by the unanimous agreement of Muslim scholars". Nevertheless, reciting Takbir loudly in a group is not Mashru` (Islamically acceptable). Rather, it is Bid`ah (innovation in religion) for it is authentically narrated that the Prophet (peace be upon him) said: He who innovates things in our affairs for which there is no valid (reason) (commits sin) and these are to be rejected. In addition, this practice was not done by our righteous predecessors; neither by the Sahabah (Companions of the Prophet) nor by the Tabi`un (Followers, the generation after the Companions of the Prophet) nor by those who followed the Tabi`un. These people are the example that Muslims should follow instead of creating Bid`ah in the Din (religion). (Part No. 8; Page No. 312) May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.