Śatapatha Brāhmaṇa ("of a hundred paths") 5.5.2.3

5.5.2.3

tadu tathā na kuryāt | ṣaḍevaitāni pūrvāṇi havīṃṣi nirvapati samānabarhīṃṣi tāsāṃ devatānāṃ rūpaṃ yathā śiśire yuktvā prāñca āprāvṛṣaṃ yāyustatṣaḍṛtūnyūṅkte ta enaṃ ṣaḍṛtavo yuktāḥ prāñca āpravṛṣaṃ vahanti ṣaḍvartūnprayuktānāprāvṛṣamanucarati pūrvāgnivāhāṃ dvau dakṣiṇā

But let him not do it thus. He prepares those first six so as to have a common barhis, after the manner of those deities (of the first six oblations); even as in early spring they would yoke their team and go onward until the rainy season, so does he now yoke the six seasons, and thus yoked the six seasons draw him forward and he follows the six seasons thus yoked until the rainy season. Two of the (oxen) drawing the original (hall-door) fire are the sacrificial fee.