Answered by Sidi Mostafa Azzam
I was on a lawful journey with a group of Muslims. They stopped to pray maghrib and isha during the beginning of isha’s time. The imam shortened his isha to two rakas. If one will finish his journey before the fajr prayer, should one shorten his isha or pray the full four rakahs?
Answer:
It is fine for you to shorten your prayer so long as you remain a traveler while you are praying it, even if you expect to arrive at your destination before that prayer’s time exits. Only if you arrive at your destination in the middle of your prayer must you complete the full four cycles (rak`ahs).
And Allah knows best.
Mostafa Azzam.