Shatapatha Brahmana 8.5.4.7

8.5.4.7

yadveva purīṣeṇa | saṃvatsara eṣo 'gnistametaccitipurīṣairvyāvartayati tadyāścatasraḥ prathamāścitayaste catvāra ṛtavo 'tha stomabhāgā upadhāya purīṣaṃ nivapati sā pañcamī citiḥ sa pañcama ṛtuḥ

And, again, as to why with loose soil;--this fire-altar is the year, and by means of the soil-coverings of the layers he divides it: those first four layers are four seasons. And having laid down the Stomabhâgâs, he throws loose soil thereon: that is the fifth layer, that is the fifth season.