Śatapatha Brāhmaṇa ("of a hundred paths") 13.6.2.17

13.6.2.17

atha vadekādaśa bhavanti ekādaśākṣarā triṣṭubvajrastriṣṭubvīryaṃ triṣṭubvajreṇaivaitadvīryeṇa yajamāno madhyataḥ pāpmānamapahate traidhātavyudavasānīyāsāveva bandhuḥ

And as to why there are eleven of them,--the Trishtubh consists of eleven syllables, and the Trishtubh is the thunderbolt, and vigour: with the thunderbolt, with vigour, the Sacrificer thus repels evil from within. The Traidhâtavî is the final offering (Udavasânîyâ): the mystic import is the same (as before).