Allah, the Exalted, says in His wise Book: Maintain the prayers and the middle prayer. What is the middle prayer? And maintain.

You ask, is this middle prayer 'asr or maghrib?

It is 'asr. That 'asr, during the time of the Messenger of Allah ﷺ.

This is from others, and among them is 'Ali, may Allah be pleased with him, that it is 'asr. This is correct, and it has been said otherwise, but the correct view is that it is 'asr, yes.