Praying Maghrib behind one who is praying ‘Isha’
Question

                I came back from a journey and I had not prayed Maghrib. I entered the mosque and found them praying ‘Isha’. Should I pray ‘Isha’ with them or pray Maghrib on my own and then pray ‘Isha’?.
Praise be to Allah.
    Rather you should join the imam in the prayer, with the 
    intention of praying Maghrib, then you should sit in the third rak’ah and 
    recite the Tashahhud and say the salaam, then join the imam in what is left 
    of ‘Isha’ prayer. Or wait during the Tashahhud until the imam finishes the 
    prayer and say the tasleem with him, then pray ‘Isha’. 

    This is the view of Imam al-Shaafa’i (may Allaah have mercy 
    on him), and is one of the two views narrated from Imam Ahmad. Al-Mardaawi 
    stated in al-Insaaf (4/413) that this view was favoured by a number 
    of the companions of Imam Ahmad, including Shaykh al-Islam Ibn Taymiyah and 
    his grandfather al-Majd ibn Taymiyah. 

    Al-Nawawi (may Allaah have mercy on him) said in 
    al-Majmoo’, 4/143: 
If he intends to pray 
    Fajr behind one who is praying Zuhr and the congregational prayer is 
    completed, if he wishes he may wait during the Tashahhud until the imam 
    finishes, and say the tasleem with him, which is better, or if he wishes he 
    may decide to leave him and say the tasleem. This leaving does not 
    invalidate his prayer, and there is no scholarly dispute on this point, 
    because he is excused from following the imam. End quote. 

    Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) was 
    asked: 

    Some worshippers were delayed in praying Maghrib, and they 
    found that the imam had started to pray ‘Isha’. Should they pray Maghrib in 
    a congregation on their own, or join the imam? And how should they pray? 

    He replied: 

    The correct view is that if a person comes (to the mosque) 
    and the imam is praying ‘Isha’, whether he is with a group or not, he should 
    join the imam, with the intention of praying Maghrib. It does not matter if 
    the imam’s intention is different from that of one who is praying behind 
    him, because the Prophet  (peace and blessings of Allaah be upon him) 
    said: “Actions are but by intentions, and each person will have but that 
    which he intended.” If they join him in the second rak’ah, they can say the 
    tasleem with him, because they will have prayed three rak’ahs, and it does 
    not matter if they sit for the Tashahhud during their first rak’ah. If they 
    join him in the first rak’ah, then when he stands up for the fourth rak’ah, 
    they can sit and recite the Tashahhud and say the tasleem, then join him in 
    what remains of ‘Isha prayer. 

    The second opinion concerning this matter is that they should 
    join him with the intention of praying ‘Isha’, then pray Maghrib after that, 
    and the obligation to offer the prayers in order is waived because of the 
    duty to join the congregation. 

    The third view is that they should pray Maghrib on their own, 
    then join him in whatever is left of ‘Isha’ prayer. The last two views 
    involve something about which there is some reservation. The first view 
    involves neglecting the order of prayers by putting ‘Isha’ before Maghrib. 
    The second view involves having two congregations in one mosque at the same 
    time, which is dividing the ummah. 

    But the first view which we mentioned is the correct one. 
    Someone may say that there is something wrong, which is saying the tasleem 
    before the imam. In fact there is nothing wrong with this. It is narrated in 
    some places in the Sunnah that a member of the congregation may do something 
    separately from the imam, such as in the fear prayer, where the imam leads 
    them in one rak’ah, then they complete the prayer by themselves and leave. 

    Another example is the story of the man who joined Mu’aadh 
    ibn Jabal (may Allaah be pleased with him) in prayer, but when he started to 
    recite Soorat al-Baqarah or another similar soorah, he left him and did not 
    complete the prayer with him.  
And the scholars said 
    that if a man is praying in congregation then he feels that is about to pass 
    wind or that he needs to break his wudoo’ by urinating or defecating, then 
    there is no sin on him if he decides to pray alone and completes his prayer 
    and leaves. This indicates that praying alone in the case of need is not 
    regarded as wrong. End quote. 

    Liqaa’aat al-Baab il-Maftooh, 
    3/425 

    Shaykh Ibn Baaz was asked: I entered the mosque and the 
    ‘Isha’ prayer had started. Before I joined the prayer, I remember that I had 
    not prayed Maghrib. Should I pray Maghrib and then catch up with ‘Isha’ with 
    the congregation, or should I pray with the congregation and then pray 
    Maghrib afterwards? 

    He replied: if you enter the mosque and ‘Isha’ prayer has 
    started, then you remember that you did not pray Maghrib, then you should 
    join the congregation with the intention of praying Maghrib. When the imam 
    stands up for the fourth rak’ah, you should sit in the third rak’ah and 
    recite the final tashahhud – i.e., al-tahhiyaatu and the blessings 
    upon the Prophet  (peace and blessings of Allaah be upon him) – and make 
    du’aa’ after that, then wait for the imam to say the tasleem and say the 
    tasleem with him. It does not matter if the imam and the member of the 
    congregation have different intentions, according to the sound scholarly 
    opinion. If you pray Maghrib on your own and then join the congregation in 
    what is left of ‘Isha’ prayer, there is nothing wrong with that. End quote. 

    Majmoo’ Fataawa Ibn Baaz, 
    12/189 
The Standing Committee 
    was asked: what should a man do if he forgets Fajr prayer, for example, and 
    does not remember until the iqaamah is given for Zuhr, or he forgets Zuhr 
    and does not remember until the time for ‘Asr begins. Should he join the 
    imam with the intention of offering the obligatory prayer that he missed or 
    with the intention of offering the prayer that is currently due, and make up 
    the missed prayer after that? 

    He replied: He should offer the prayer that he forgot behind 
    the imam, and it does not matter if his intention is different from that of 
    the imam, according to the sound scholarly opinion. End quote. 

    Fataawa al-Lajnah al-Daa’imah, 
    7/407.