What is the ruling regarding an Imam who stood up to add two rak’ahs after the last rak’ah of Isha prayer without performing the tasleem, and are the congregants obliged to follow him while remaining in their places?

If the Imam forgets to say the taslim (salutation), the congregants should alert him by saying SubhanAllah (Glory be to Allah). If he does not respond, the congregants should intend to separate from him and say the taslim, and their prayer will be valid. As for the Imam, he must repeat the prayer because he deliberately omitted a pillar, unless he is a Hanafi, as they do not consider the taslim obligatory. This is a weak opinion because it contradicts the actions of the Prophet, peace and blessings be upon him, and his saying: "Pray as you have seen me praying," and his saying: "The key to prayer is purification, its consecration is takbir (pronouncing Allahu Akbar), and its deconsecration is taslim." If the Imam omits the taslim for this reason, then what the congregants did by saying the taslim and departing is correct, because the Imam's prayer is complete in his view, but the congregants' prayer is not complete except with the taslim. The Imam should be advised and the Sunnah explained to him.