In Ramadan, we find some of the worshippers praying the obligatory 'Isha' prayer behind the imam who is praying the tarawih; is this correct?

The questioner : There is a question, O our Shaykh; in Ramadan, we find some of the worshippers praying the obligatory 'Isha' prayer behind the imam who is praying two rak'ahs, meaning he is praying the tarawih, praying the two rak'ahs of tarawih; is this action praiseworthy or blameworthy? And what is the evidence for that?

The Shaykh : If it is intentional, then it is naturally not permissible, not just blameworthy.

The questioner : Yes.

The Shaykh : Because it means he is intentionally not praying with the congregation. But if it is an oversight, then it is permissible.