He remembered the 'Asr Prayer at the Time of the Call to Maghrib and so He delayed it until after Maghrib

Question :

We forgot to pray 'Asr and we did not remember it until we heard the Athan for Maghrib, so we prayed 'Asr after performing the Maghrib prayer. What is the ruling on this?

Answer:

If a person forgot to perform a prayer, or he slept through it and there was nobody to wake him or remind him until the time for it had ended, then he should do as the Prophet said:
"He must make up for it as soon as he remembers it, and there is no expiation for it except this.

In such circumstances as those which befell the questioner, he should begin first with the 'Asr prayer and then Maghrib, so as to maintain the correct order, in accordance with what Allah, the Almighty, the All-Powerful has ordained. This is because when the Prophet missed prayers on one of the days of the battle of the Trench, he made them up in order. And it has been authentically reported from him that he said:
Pray as you have seen me praying.Based upon this, when you came to the mosque while the people were praying Maghrib, you should have joined them with the intention of 'Asr prayer. Then when the Imam made the Taslim for the Maghrib prayer, you should have stood up and completed the remainder of the Asr prayer; thus the prayer would have been Maghrib for the congregation, but 'Asr for you. There is no harm in this I mean the difference between the intention of the Imam and that of the worshipper, because the actions are the same, and the differing which the Prophet forbade between the Imam and the worshippers was in the actions, not the intention. But since what you did, performing the Maghrib prayer before 'Asr was done out of ignorance, there is no harm for you in it.

Source:
Ibn 'Uthaimin
Fatawa Islamiyah, Vol. 2 Pages 244-245