Bukhari

Explanation:
The Bedouins used to refer to the Maghrib prayer as ‘Isha and the ‘Isha prayer as ‘Atamah. Therefore, the Noble Prophet sallallahu alayhi wa sallam said that the terminology of the Bedouins should not prevail; rather, these prayers should be called by their proper names, Maghrib and ‘Isha. ‘Atamah refers to the remaining milk that stays in the udder of a she-camel and is extracted after a portion of the night has passed. Some have said that ‘Atamah means to delay until the darkness of the night, and since this is the time for the ‘Isha prayer, it was called ‘Atamah. On some occasions, the ‘Isha prayer has been referred to as Salat al-‘Atamah, and thus it has been given the status of permissibility. However, it is better that it be referred to by the name ‘Isha.

Hafiz Ibn Hajar rahimahullah states that this prohibition was made because the meaning of ‘Isha in the language is “darkness,” and this occurs after the disappearance of twilight. Thus, if the Maghrib prayer were to be called ‘Isha, there is a possibility that in the future people would begin to think that the time for Maghrib is after the disappearance of twilight.

Hadith Commentary:
(1)
The Bedouins used to adopt an opposite method in naming. They would call Maghrib (sunset prayer) as ‘Isha, whereas Maghrib refers to the very beginning of sunset, and ‘Isha refers to the early darkness of the night, which begins when the twilight disappears. That is, giving the name of the last time to the first time leads to great confusion; therefore, the Shari‘ah has corrected this. This is why it is not established from any hadith that the Messenger of Allah (sallallahu alayhi wa sallam) ever applied the term ‘Isha to Maghrib. If ‘Isha is applied to Maghrib, there is a risk that the beginning time of Maghrib will be considered as the disappearance of the twilight. This is also prohibited because it has two harms as follows:
* The preservation of the Islamic language will not be possible.
* There will be confusion in the rulings, whereas the rulings for both are separate.
If there is no confusion in the rulings and there is no fear of the dominance of the Bedouin language, then it will not be prohibited. For example, if Maghrib is called ‘Isha al-Ula (the first ‘Isha) and ‘Isha is called al-Akhirah (the last), or both are called ‘Ishayan (the two ‘Ishas) by way of predominance.

(2)
The purpose of Imam Bukhari (rahimahullah) is that, in the explanation of the legal rulings, the use of Shari‘ah terminology should be adhered to. Maghrib should be referred to by its own name, and the term ‘Isha should not be applied to it, because by doing so, the name of the pre-Islamic era would prevail over the Islamic name, and there would also be confusion in the legal rulings. (Fath al-Bari: 2/58)