Does a traveler join a congregation praying Maghrib with the intention of 'Isha' if he has already prayed Maghrib?

Question:
Honorable Shaykh, may Allah grant you success. He says: A traveler entered the mosque and found a congregation praying Maghrib, and he had already prayed it. He wants to pray 'Isha'. Does he join them with the intention of 'Isha', or is it permissible for him to establish another congregation?

Answer:
He joins them with the intention of 'Isha' and completes it. When the imam says the closing salam, he stands and performs the fourth rak'ah. There is no harm in this. He is acting according to his intention, for actions are by intentions. His aim is to catch the congregation rather than praying alone. There is no harm in this, if Allah wills.