Fath al-Rabbani

Benefits: … By "prohibition" (تحریم) is meant that all those actions become forbidden which are impermissible during prayer, and by "permissibility" (تحلیل) is meant that those actions which had become forbidden due to the prayer now become permissible. From this hadith, it is understood that to enter into prayer, only "Allahu Akbar" will be said, and to exit from prayer, only salam will be said. Since in the last two sentences mentioned in the blessed hadith, the predicate precedes and the subject comes after, exclusivity is established in the meaning.

Imam Abu Hanifah rahimahullah is of the opinion that prayer can be started with any phrase in which there is glorification of Allah, for example: "Ar-Rahmanu Akbar" (The Most Merciful is the Greatest), "Allahu Ajall" (Allah is the Most Majestic), "Allahu A'zam" (Allah is the Most Supreme). However, in light of the ahadith on this subject, this opinion is not correct. The majority of scholars, both from the early and later generations, have not agreed with this opinion. From the statements, actions, and commands of the Prophet sallallahu alayhi wa sallam, only the phrase "Allahu Akbar" is established. For example: It is narrated from Sayyidah Aishah radi Allahu anha that when the Messenger of Allah sallallahu alayhi wa sallam would begin prayer, he would say the takbir (i.e., "Allahu Akbar"). (Sahih Muslim: 498)

Sayyiduna Abu Humayd As-Sa'idi radi Allahu anhu reports that when the Messenger of Allah sallallahu alayhi wa sallam would stand for prayer, he would face the qiblah, raise his hands, and say "Allahu Akbar." (Ibn Majah: 803)

It is narrated from Sayyiduna Abu Hurairah radi Allahu anhu that a man entered the mosque, prayed, then came to the Prophet sallallahu alayhi wa sallam and greeted him with salam. The Prophet sallallahu alayhi wa sallam returned the greeting and said: ((Irji‘ fa salli fa innaka lam tusalli)) — "Go back and pray, for you have not prayed." This happened three times. Finally, the man requested, "O Messenger of Allah! Teach me how to pray." The Prophet sallallahu alayhi wa sallam said: ((Iza qumta ila as-salati fa asbigh al-wudu’, thumma istaqbil al-qiblah fa kabbir …)) — "When you intend to pray, perform complete ablution (wudu), then face the qiblah and say the takbir (i.e., 'Allahu Akbar') … ." (Sahih Bukhari: 6251, Sahih Muslim: 397, Sunan Arba‘ah)

According to the narration of Sayyiduna Rifa‘ah ibn Rafi‘ radi Allahu anhu, the Prophet sallallahu alayhi wa sallam said regarding the one who prayed incorrectly: ((Innahu la tatimmu salatun li ahadin min an-nasi hatta yatawadda’, thumma yukabbir …)) — "No one's prayer is complete until (he performs these actions): he performs ablution, then says the takbir (i.e., 'Allahu Akbar') … ." (Abu Dawud: 857)

The Noble Prophet sallallahu alayhi wa sallam began the prayer with "Allahu Akbar" and said: ((Sallu kama ra’aytumuni usalli.)) — "Pray as you have seen me pray." (Sahih Bukhari: 631)

Ibn Humam al-Hanafi considers beginning the prayer with "Allahu Akbar" to be obligatory. (Fath al-Qadir: 1/284)

There are other evidences as well, so there is no need to delve into any further opinions.