Śatapatha Brāhmaṇa ("of a hundred paths") 7.1.1.32

7.1.1.32

aṣṭāviṣṭakā upadadhāti | aṣṭākṣarā gāyatrī gāyatro 'gniryāvānagniryāvatyasya mātrā tāvantamevainametaccinoti pañca kṛtvaḥ sādayati pañcacitiko 'gniḥ pañca 'rtavaḥ saṃvatsaraḥ saṃvatsaro 'gniryāvanagniryāvatyasya mātrā tāvantamevainametaccinotyaṣṭāviṣṭakāḥ pañca kṛtvaḥ sādayati tattrayodaśa trayodaśa māsāḥ saṃvatsarastrayodaśāgneścitipurīṣāṇi yāvānagniryāvatyasya mātrā tāvattadbhavati

Eight bricks he puts on (the hearth-site),the Gâyatrî consists of eight syllables, and Agni is Gâyatra: as great as Agni is, as great as is his measure, so great he thus builds him. Five times he 'settles' (the bricks)--the fire-altar consists of five layers; five seasons are a year, and Agni is the year: as great as Agni is, as great as is his measure, so great he thus builds him. Eight bricks he 'settles' five times, that makes thirteen,--thirteen months are a year, and there are thirteen layers of earth in the fire-altar: as great as Agni is, as great.as is his measure, so great does this become.