Is the prayer valid for someone who missed the 'Isha prayer and joined the Imam who is praying Tarawih, intending to pray 'Isha, then the Imam said the taslim after two rak'ahs and the worshipper remained seated to stand with the Imam when he stands to complete his prayer?

Scholars differed concerning the ruling on the obligatory prayer behind an Imam performing a supererogatory prayer. The preponderant opinion is that it is permissible, and this is the view of Ash-Shafi'i and Ibn Al-Mundhir, and it was chosen by the scholars of the Permanent Committee and Sheikh Ibn Baz.

As for completing the prayer with the Imam for two additional rak'ahs after the Imam's taslim: Sheikh Al-'Uthaymeen paused on this matter, while An-Nawawi favored its permissibility.

Therefore, the questioner's prayer is valid, and he is not required to repeat it. However, it is preferable for him to complete his prayer alone after the Imam's taslim. If he follows the Imam for the two additional rak'ahs, that is permissible.