Is it permissible to join a group praying Maghrib, intending to pray 'Isha' shortened?

Question:
May Allah reward you, Honorable Shaykh. This questioner says: We were a group traveling, and when we stopped to pray, we prayed Maghrib first. When we finished the prayer, another group came and prayed Maghrib. We wanted to pray 'Isha' with them. One of us joined the imam from the first rak'ah, and when the imam finished the second rak'ah, he separated from him. As for me, I waited until the imam prayed one rak'ah, then I joined him for the remaining two rak'ahs. The question is: Who is correct?

Answer:
You are all mistaken. You are all mistaken in this action. You do not enter the Maghrib prayer with the intention of shortening. If you enter with the imam in the Maghrib prayer, you complete four rak'ahs. When he says the taslim from the third, you stand up and perform the fourth. You do not shorten behind the Maghrib prayer. And your praying on your own, shortening, is better than your joining the imam. But if you join, you complete the prayer. When the imam says the taslim from the third, you stand up and perform the fourth. Do not shorten behind the Maghrib. But what happened was due to ignorance. We hope that your prayer is valid, but do not return to something like this. Yes.