What is the ruling on the prayer of a congregant who did not make the taslim in the first rak'ah after the imam made a mistake and offered the taslim in it, and then waited for the imam to complete the second rak'ah, and performed the prostrations of forgetfulness (sujud al-sahw) with him, knowing that the Tarawih prayer is a supererogatory (nafilah) prayer?

If the Imam makes the intention for two rak'ahs and then intends to suffice with one rak'ah and makes the taslim, then performs another, this is permissible according to some scholars, provided the intention is changed before performing the rak'ah. The prostration of forgetfulness (sujud al-sahw) here is out of place, but the prayer is not invalidated by performing it out of ignorance or forgetfulness. The follower (ma'mum) who did not follow the Imam in this error and waited for him to complete the rak'ah was correct, and separating from the Imam in this case is valid. The follower should wait for the Imam if he is certain of his mistake because the Imam's prayer is valid.