What does a traveler do if he wants to pray Maghrib, enters the mosque, and finds the people praying 'Isha'?

The questioner: O Shaykh, if I missed the Maghrib prayer while traveling or something like that, and I came to the mosque and attended the 'Isha prayer.

The Shaykh : Yes.

The questioner: Do I enter intending Maghrib, or?

The Shaykh: It is obligatory to have this intention; it is obligatory to pray Maghrib before 'Isha. If you catch the imam in the first rak'ah, then when he stands for the fourth rak'ah, you intend to separate from him, sit in the third rak'ah, recite the tashahhud, and say the taslim. Then you stand up and catch the imam either in the last part of the fourth rak'ah of the 'Isha prayer, in which case you intend to pray 'Isha with this imam. In any case, wherever you catch the imam, you intend to pray 'Isha. When the imam says the taslim, you stand up and complete your prayer. Is that clear?

The questioner: And if I catch the imam in the third rak'ah of 'Isha, do I pray Maghrib intending Maghrib?

The Shaykh: It is obligatory to start with Maghrib in any of the scenarios.

The questioner: Ah, may Allah reward you with good.

The Shaykh : And beware of that.