What is the ruling on interrupting the 'Asr prayer after three rak'ahs and then completing it individually, due to a person joining in congregation who thought it was the Maghrib prayer?

If a worshipper intentionally says the Taslim before completing their prayer, then their prayer becomes invalid and must be made up, whether it is the Asr prayer or any other. This is because an intentional Taslim before its proper place is one of the nullifiers of prayer, unlike an unintentional Taslim due to forgetfulness, which does not nullify it. The questioner was obliged to complete their prayer with four rak'ahs. As for the follower who is praying Maghrib, their prayer is valid according to some scholars.