A traveller prays ‘Isha’ behind a resident who is praying Taraweeh – should he shorten his prayer or offer it in full?
Question

                A man who was travelling had not prayed ‘Isha’. He entered the mosque when the imam was praying Taraweeh and was still in the first rak’ah. The traveller joined the prayer with the imam with the intention of praying ‘Isha’. When the imam says the salaam after two rak’ahs, should the traveller complete the prayer based on the ruling that the imam is a resident, or should he say the salaam on the basis that he is a traveller?.
Praise be to Allah.The correct scholarly view 
    is that he should say the salaam with the imam, because then he has followed 
    the Sunnah by shortening his prayer, and he has not gone differ from the 
    imam, because the imam prayed two rak’ahs. 
The traveller is only 
    enjoined to offer the prayer in full with four rak’ahs if the imam is 
    praying four rak’ahs, so that he will not be differing from him. Shaykh Ibn 
    ‘Uthaymeen (may Allaah have mercy on him) was asked: I entered al-Masjid 
    al-Haraam to pray ‘Isha’ and I found him praying Taraweeh. Should I pray 
    Taraweeh with the intention of ‘Isha’, or should I pray ‘Isha’? 
He replied: If a person 
    enters the mosque and the people are praying Taraweeh, and he has not prayed 
    ‘Isha’ yet, then he should join them with the intention of praying ‘Isha’, 
    then if he is travelling and he joined the imam in the first rak’ah, he can 
    say the salaam with the imam, because the traveller prays two rak’ahs. If he 
    is a resident, then when the imam says the salaam he should complete 
    whatever he still has to do of the four rak’ahs. Someone may say: How can he 
    offer an obligatory prayer behind someone who is offering a naafil prayer? 
    But this happened at the time of the Prophet  (peace and blessings of 
    Allaah be upon him), as Mu’aadh ibn Jabal (may Allaah be pleased with him) 
    used to pray ‘Isha’ with the Prophet  (peace and blessings of Allaah be 
    upon him), then he would go to his own people and lead them in the same 
    prayer – it was naafil for him and obligatory for them. Even if there is 
    someone with him, it is better for them not to be separate from the other 
    Muslims; it is better for them to pray with the Muslims and intend their 
    prayer to be the obligatory one. End quote from Liqa’ al-Baab il-Maftooh 
    (30/117). 
And Allaah knows best.