Śatapatha Brāhmaṇa ("of a hundred paths") 5.1.4.9

5.1.4.9

atha savyāyugyaṃ yunakti | vātaraṃhā bhavavājinyujyamāna iti vātajavo bhava vājinyujyamāna ityevaitadāhendrasyeva dakṣiṇaḥ śriyaidhīti yathendrasya dakṣiṇaḥ śriyaivaṃ yajamānasya śriyaidhītyevaitadāha yuñjantu tvā maruto viśvavedasa iti yuñjantu tvā devā ityevaitadāha te tvaṣṭā patsu javaṃ dadhātviti nātra tirohitamivāstyatha dakṣiṇāpraṣṭiṃ yunakti savyāpraṣṭiṃ vā agre mānuṣe 'thaivaṃ devatrā

He then yokes the left horse, with (Vâg. S. IX, 8), 'Become thou swift as the wind, O courser, being yoked!'--thereby he says, 'Become quick as the wind, O courser, being yoked;'--'be thou as Indra's right (steed) in beauty!'--he thereby says, 'Even as Indra's right (steed) for beauty, so be thou that of the sacrificer for beauty!'--'May the all-knowing Maruts yoke thee!' he thereby says, 'may gods yoke thee!'--'May Tvashtri lay speed into thy feet!' in this there is nothing obscure. He then yokes the right side-horse; for in human (practice) they indeed yoke the left side-horse first, but with the gods in this way.