Should the traveler complete the 'Isha prayer after the Imam has concluded the Tarawih prayer, or should he conclude it with him, knowing that his intention was to pray 'Isha?

It is permissible for someone who entered the mosque and has not prayed 'Isha to pray Tarawih with the Imam. He must then complete his prayer as four rak'ahs, because he is following an Imam who is praying a complete prayer. It was reported from Ibn 'Umar and Ibn 'Abbas that a traveler completes his prayer as four rak'ahs if he prays behind a resident.