The ruling on a traveler praying behind the Maghrib prayer with the intention of 'Isha'

Question:
We have a call from our brother 'Abd al-Karim from al-Kharj. May Allah welcome you. Peace be upon you, and Allah's mercy and blessings. Peace and Allah's mercy. The question, Shaykh: A traveler prayed Maghrib in three rak'ahs and wanted to shorten the prayer. After he finished the Maghrib prayer, he found a congregation establishing the prayer behind him. So he waited until they prayed the first rak'ah, then he joined them in the last two rak'ahs and prayed with them with the intention of 'Isha', shortened. Is this prayer of his valid or not? And what is the stronger position in this matter?

Answer:
This prayer of his is not valid, because Maghrib is not shortened. The one who prays behind Maghrib does not shorten; he completes the prayer because he is praying behind a prayer that is not shortened. Likewise, the one who prays behind an imam who prays the four-rak'ah prayer and completes it, the ma'mum completes the prayer behind him. It is obligatory for him to follow the imam. He does not pray two rak'ahs behind Maghrib. And he does not pray two rak'ahs behind an imam who completes the prayer, because he is not traveling, or because he completed the prayer while traveling. So his ruling is the ruling of the imam. Yes.

The presenter:
Thank you, my brother 'Abd al-Karim. May Allah bless you. May Allah welcome you.