What is the ruling on the prayer of someone who, by mistake, said the Tasleem in the final Tashahhud before the Imam?

If the congregant (ma'mum) pronounces the salām inadvertently behind the imam, his prayer is valid if he immediately returns to following the imam and then pronounces the salām after his imam. However, if he contents himself with this salām before the imam, his prayer is void, because preceding the imam in the opening takbir (ihrām) and the salām invalidates the prayer.