What is the ruling on vocalizing the recitation in the third rak'ah after intending to complete the solo prayer as a congregational one, then completing it as three rak'ahs?

If someone intentionally adds an extra rak'ah (unit of prayer) while knowing it is forbidden, their prayer is invalid and must be repeated. However, if the addition was due to ignorance or forgetfulness, their prayer is valid. Increasing the volume of recitation in a place where it is not prescribed does not invalidate the prayer. The prayer of the follower is valid and is not harmed by the invalidation of the Imam's prayer.