Śatapatha Brāhmaṇa ("of a hundred paths") 6.8.1.8

6.8.1.8

athānaḍvāhau yunakti | dakṣiṇamagre 'tha savyamevaṃ devatretarathā mānuṣe sa yāṃ kāṃ ca diśaṃ yāsyantsyātprāṅevāgre prayāyātprācī hi digagneḥ svāmeva taddiśamanu prayāti

He then yokes two oxen, first the right one, then the left one: so (it is done) with the gods, otherwise in human (practice). And in whatever direction he may intend to drive, let him first drive east, for the east is Agni's region: he (Agni) thus proceeds towards his own region.