Śatapatha Brāhmaṇa ("of a hundred paths") 3.5.1.7

3.5.1.7

atha yattriṃśadvikramā paścādbhavati | triṃśadakṣarā vai virāḍvirājā vai devā asmiṃloke pratyatiṣṭhaṃstatho evaiṣa etadvirājaivāsmiṃloke pratitiṣṭhati

Now the reason why it is thirty steps broad behind is this: the Virâg metre consists of thirty syllables, and by means of the Virâg the gods obtained a firm footing in this world; and even so does he now, by means of the Virâg, obtain a firm footing in this world.