Is the opening takbir (Allahu Akbar) for prayer recited only in the first rak'ah (unit of prayer) or in all rak'ahs?

The opening takbir (Allahu Akbar) is a pillar of prayer, and the prayer is not valid without it. It is named this way because it prohibits actions that were permissible before it. It is specific to the first rak'ah only and is not repeated in the other rak'ahs.