Śatapatha Brāhmaṇa ("of a hundred paths") 8.6.3.12

8.6.3.12

aṣṭāviṣṭakā upadadhāti | aṣṭākṣarā gāyatrī gāyatro 'gniryāvānagniryāvatyasya mātrā tāvataivainametadreto bhūtaṃ siñcati pañca kṛtvaḥ sādayati pañcacitiko 'gniḥ pañca 'rtavaḥ saṃvatsaraḥ saṃvatsaro 'gniryāvānagniryāvatyasya mātrā tāvataivainametadreto bhūtaṃ siñcatyaṣṭāviṣṭakāḥ pañca kṛtvaḥ sādayati tattrayodaśa trayodaśa māsāḥ saṃvatsarastrayodaśāgneścitipurīṣāṇi yāvānagniryāvatyasya mātrā tāvattadbhavati

He lays down eight bricks,--the Gâyatrî (metre) consists of eight syllables, and Agni (the fire-altar) is of Gâyatrî nature: as great as Agni is, as great as is his measure, so great he thus introduces him in the form of seed. Five times he 'settles' it,--of five layers consists the fire-altar, five seasons make a year, and Agni is the year: as great as Agni is, as great as is his measure, by so much he thus introduces him in the form of seed. Eight bricks he 'settles' five times, that makes thirteen,--thirteen months make a year, and there are thirteen 'layer-fillings' of the altar: as great as Agni is, as great as is his measure, so great he thus becomes.