Śatapatha Brāhmaṇa ("of a hundred paths") 10.2.3.2

10.2.3.2

te vā ete vyāmaikādaśāḥ prakramā antarā vedyantaṃ ca gārhapatyaṃ caikādaśākṣarā triṣṭubvajrastriṣṭubvīryaṃ triṣṭubvajreṇaivaitadvīryeṇa yajamānaḥ purastādyajñamukhādrakṣāṃsi nāṣṭrā apahanti

Now, there are here, including the fathom (as one), eleven steps between the end of the Vedi and the (original) Gârhapatya;--the Trishtubh consists of eleven syllables, and the Trishtubh is a thunderbolt, and the Trishtubh means strength: it is thus by the thunderbolt, and by strength, that the Sacrificer from the very first drives off the fiends, the Rakshas, from the mouth of the sacrifice.