What is the ruling regarding the 'Isha prayer if the Imam stood up after two rak'ahs without the tashahhud, then he was alerted, so he sat down, completed the prayer, and performed the prostration of forgetfulness after the taslim?

The preponderant opinion among scholars is the validity of the worshipper's prayer if he forgets and returns after standing upright in the third rak'ah and before starting to recite Al-Fatihah, even though he has committed an error. The ma'mum (follower) must follow the imam if he stands up and does not return, or if he returns to the tashahhud before recitation. He should perform the prostration of forgetfulness (sujud al-ba'di) according to the Maliki school when he returns. What the imam did by prostrating after the taslim is valid according to the Maliki school.