What is the ruling on the prayer of someone who unintentionally said the salām before the Imām in congregational prayer?

If a worshipper (ma'mum) pronounces the salutation (taslim) inadvertently behind the imam, then if he returns to his imam and follows him until the imam pronounces the salutation, there is nothing upon him. However, if he does not return to his imam, then his prayer becomes null and void due to the obligation of following the imam.