Ruling on the traveler praying behind one performing a voluntary prayer

Question:
The brother M. A. A. from Shaqra says in his question: A traveling man entered the mosque to perform the 'Isha' prayer, and the imam was praying tarawih. So he joined him with the intention of praying 'Isha' shortened. Is his action valid, O Shaykh?

Answer:
There is no blame on him for that; because it is established from the Prophet (peace and blessings be upon him) what indicates the permissibility of one who has an obligatory prayer praying behind one who is performing a voluntary prayer. And because the traveler’s prayer is two rak'ahs, and the tarawih prayer is two rak'ahs, so there is no difference in the number.
As for if the traveler prays behind a resident who is praying the Zuhr, 'Asr, or 'Isha' prayer, it is obligatory for him to pray four rak'ahs, even if he does not catch with the imam except part of the prayer; because the Sunnah is authentic from the Prophet (peace and blessings be upon him) regarding that. And success is from Allah.