Śatapatha Brāhmaṇa ("of a hundred paths") 7.2.3.9

7.2.3.9

trayodaśaitā vyāhṛtayo bhavanti | trayodaśa māsāḥ saṃvatsarastrayodaśāgneścitipurīṣāṇi yāvānagniryāvatyasya mātrā tāvantamevainametaccinotyājyena juhotyagnireṣa yadājyamagnimevaitaccinoti pañcagṛhītena pañciciko 'gniḥ pañca 'rtavaḥ saṃvatsaraḥ saṃvatsaro 'gniryāvānagniryāvatyasya mātrā tāvantamevainametaccinotyūrdhvāmudgṛhṇanjuhotyūrdhvaṃ tadagniṃ citibhiścinoti

There are thirteen utterings,--thirteen months are a year; thirteen in number are the layers of bricks and earth of the fire-altar: as great as Agni is, as great as is his measure, so great he thus builds him up. With butter he sacrifices,--butter is the same as Agni: it is Agni he thus builds up. With fivefold-taken (butter he offers),--the altar consists of five layers,--five seasons are a year, and the year is Agni: as great as Agni is, as great as is his measure, so great he thus builds him up. He offers raising (the spoon) upwards: he thus builds Agni upwards by means of the layers (of the altar).