Śatapatha Brāhmaṇa ("of a hundred paths") 5.5.5.6

5.5.5.6

tasya yo yonirāśaya āsa | tamanuparāmṛśya saṃlupyācinatsaiṣeṣṭirabhavattadyadetasminnāśaye tridhāturivaiṣā vidyāśeta tasmāttraidhātavī nāma

And that which had been his (Vritra's) seat, his retreat, that he shattered, grasping it and tearing it out: it became this offering. And because the science (the Veda) that lay in that retreat was, as it were, a threefold (tridhâtu) one, therefore this is called the Traidhâtavî (ishti).