Śatapatha Brāhmaṇa ("of a hundred paths") 1.3.4.10

1.3.4.10

atha stīrṇāṃ vedimupāvartate | sa dve tṛṇe ādāya tiraścī nidadhāti saviturbāhū stha ityayaṃ vai stupaḥ prastaro'thāsyaite bhruvāveva tiraścī nidadhāti tasmādime tiraścyau bhruvau kṣatraṃ vai prastaro viśa itaram barhiḥ kṣatrasya caiva viśaśca vidhṛtyai tasmāttiraścī nidadhāti tasmādveva vidhṛtī nāma

He now returns to the altar covered (with sacrificial grass). Having taken two stalks of grass, he lays them down across (the barhis or grass covering, with the tops to the north), with the formula (Vâg. S. II, 5), 'Savitri's arms are ye!' The prastara bunch is indeed the top-knot (of the sacrifice); and he now lays down these two crosswise as its eye-brows: thereby these two (represent) the transverse eye-brows. The prastara, further, (represents) the kshatra (or military class); and the other barhis the vis (or, the common Âryan people);--(and the two stalks he puts down between them) for the sake of separating (vidhriti) the kshatra and the vis: for this reason he lays them down crosswise; and for this reason these two (stalks) are called vidhriti.