Shatapatha Brahmana 5.5.2.4

5.5.2.4

ṣaḍevottarāṇi havīṃṣi nirvapati | samānabarhīṃṣi tāsāṃ devatānāṃ rūpaṃ yathā punarāvarteranvārṣikamabhi tatṣaḍṛtūnyuṅkte ta enaṃ ṣaḍṛtavo yuktā vārṣikamabhi vahanti ṣaḍvartūnprayuktānvārṣikamanucarati pūrvāgnivāhāṃ dvau dakṣiṇā tadyatpūrvāgnivāho dakṣiṇa 'rtūnvā etatsuṣavāṇo yuṅkte vahanti vā anaḍvāhastasmātpūrvāgnivāho dakṣiṇā

He prepares the last six oblations so as to have a common barhis, after the manner of those (six) deities. Even as they would return again towards the rainy season, so does he yoke the six seasons, and thus yoked the six seasons draw him towards the rainy season, and he follows the six seasons thus yoked, in the rainy season. Two of the (oxen) drawing the original fire are the sacrificial fee. And as to why the (oxen) drawing the original fire are the sacrificial fee,--the consecrated (king) now yokes the seasons, and it being oxen that (actually) draw (and thus represent the seasons), therefore the (oxen) drawing the original fire are the sacrificial fee.