Is it prescribed for the worshipper to repeat the prayer or perform the prostration of forgetfulness alone if the Imam adds an extra rak'ah and then requests the prayer to be repeated?

If the Imam adds an extra rak'ah due to forgetfulness, his prayer is not invalidated, nor is the prayer of those praying behind him, provided that the followers do not intentionally follow him while knowing about his addition and the impermissibility of following him in that. The followers are obligated to perform the prostration of forgetfulness (sujud al-sahw) with the Imam. If the Imam does not perform it, the follower should perform it individually.