Why do we begin the prayer with "Allahu Akbar" and not any other of Allah's Most Beautiful Names, and why do we repeat it in the rest of the prayer's actions?

The Takbirat al-Ihram (the opening Takbir) is one of the pillars of prayer (Salah); the prayer is not valid without it, as the Prophet, peace and blessings be upon him, said: "The key to prayer is purification (Taharah), its consecration (Tahrim) is the Takbir, and its desacralization (Tahlil) is the Taslim." Its form is to say: "Allahu Akbar (Allah is the Greatest)," and this is the view of the Malikis, Shafi'is, Hanbalis, and the general body of scholars. This is contrary to Abu Hanifa, who believes that prayer can be valid with any remembrance purely for Allah Almighty, intended to magnify Him.

The correct view is that the prayer is not valid except by saying: "Allahu Akbar," due to its affirmation by the Prophet, peace and blessings be upon him, through both verbal and practical Sunnah. The general principle for acts of worship is that they are prescriptive (Tawqifiyyah), meaning they must be performed exactly as prescribed.

Takbirat al-Ihram is a pillar without which the prayer is not valid. As for the Takbirs of transition (Takbirat al-Intiqal), they are obligatory according to some scholars, and Sunnah according to the majority. The repetition of these Takbirs in the known form is indicated by the Hadith of Abu Hurairah and others, and by the Prophet's saying, peace and blessings be upon him: "Pray as you have seen me pray."