If he prays ‘Isha’ with Maghrib at the time of Maghrib, he can pray Witr immediately after that.

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One of our brothers/sisters has asked this question:
If I join Maghrib and ‘Isha’ at the time of Maghrib because of traveling, when should I pray Witr? Should I pray it straight after ‘Isha’ or wait until the time for ‘Isha’ begins?.
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Answer:
Praise be to Allaah.

You can pray it straight after praying ‘Isha’. 

It says in Fataawa al-Lajnah al-Daa’imah (8/144): 

It is prescribed for travellers and those who are sick to join prayers, and also for non-travellers to join prayers on a rainy night. They may then pray Witr immediately after praying ‘Isha’ when it is joined to Maghrib at the time of Maghrib.

Whatever written of Truth and benefit is only due to Allah's Assistance and Guidance, and whatever of error is of me. Allah Alone Knows Best and He is the Only Source of Strength.

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