He followed his imam in Taraweeh to pray ‘Isha’, then he followed him again for the last two rak’ahs
Question

                I missed ‘Isha’ prayer and the imam began to pray Taraweeh. I joined the prayer with the imam with the intention of praying ‘Isha’. The imam prayed two rak’ahs then he said the salaam, and I remained sitting but did not say the salaam. When he stood up to pray again I stood up with him and completed my ‘Isha’ prayer with him. Is the way I offered this prayer correct? If it was not correct, what should I do?.
Praise be to Allah.
    The scholars differed concerning the ruling on offering an 
    obligatory prayer behind an imam who is offering a naafil prayer. We have 
    quoted some of the scholarly opinions on this matter in the answer to 
    question number 79136.  

    Imam al-Shafaa’i and Ibn al-Mundhir are of the view that this 
    is permissible, and this is also narrated from Ahmad. The scholars of the 
    Standing Committee and Shaykh Ibn Baaz also favoured this view, and in the 
    question mentioned above we quoted him as saying that it is permissible, 
    namely praying ‘Isha’ behind one who is praying Taraweeh, and that the 
    person praying behind him should complete his prayer on his own after the 
    imam says the salaam. 

    As for what the questioner did, sitting after the imam said 
    the salaam so that he could follow him in the next two rak’ahs of Taraweeh, 
    there are two scholarly opinions concerning the one who starts his prayer on 
    his own -- can he follow an imam who is leading a congregation? Some of them 
    said that this is not allowed and some of them said that it is valid. 

    Shaykh al-‘Uthaymeen did not give a definitive opinion 
    concerning this action and he said, after stating that it is permissible for 
    one who is offering an obligatory prayer to pray behind one who is offering 
    a naafil prayer, which includes it being permissible to offer ‘Isha’ prayer 
    behind one who is praying Taraweeh: 

    Rather what I am not sure about is their waiting for the imam 
    until he starts the next two rak’ahs and completing the prayer with him. 
    This is what I am not sure about, because the Messenger (blessings and peace 
    of Allah be upon him) said: “Whatever you catch up with, pray, and whatever you miss, complete it.” The apparent meaning is that a person should complete what he missed with his imam on his own, meaning that he should not 
    wait until the imam starts another prayer. Rather we say: When the imam says 
    the salaam at the end of the prayer in which you caught up with him, then 
    complete the prayer and do not wait until he starts another prayer. End 
    quote. 

    Fataawa Noor ‘ala al-Darb (Tape 
    no. 15, side B) 

    Al-Nawawi thought it more likely that this is permissible, as 
    he said: 

    If he prays ‘Isha’ behind Taraweeh, that is permissible, and 
    when the imam says the salaam, he should stand up to do the remaining two 
    rak’ahs. It is better for him to complete them on his own. If the imam gets 
    up to do two more rak’ahs of Taraweeh, he should form the intention to 
    follow him again in his two rak’ahs. There are two opinions as to whether it 
    is permissible for one who started the prayer on his own to then intend to 
    follow the imam. The more correct view is that it is valid. End quote. 

    Al-Majmoo’ Sharh al-Muhadhdhab, 
    4/270 

    Based on this, the prayer was valid and you do not have to 
    repeat it, but it is better -- in the future -- to complete your prayer on 
    your own and not to join with the imam again. 

    The brother who asked this question does not have to repeat the prayer or to repeat the two rak’ahs. The ‘Isha’ prayer that he offered behind the imam who was praying Taraweeh is valid. But we think that it is better for him to complete what remains for him of his prayer on his own. But if he follows the last two rak’ahs with the imam again, that is permissible. 

    And Allah knows best.