Shatapatha Brahmana 6.7.1.27

6.7.1.27

athātaḥ sampadeva | āsando cokhā ca śikyaṃ ca rukbhapāśaścāgniśca rukbhaśca tatṣaṭ ṣaḍṛtavaḥ saṃvatsaraḥ saṃvatsaro 'gniryāvānagniryāvatyasya mātrā tāvattadbhavatīṇḍve tadaṣṭāvaṣṭākṣarā gāyatrī gāyatro 'gniryāvānagniryāvatyasya mātrā tāvattadbhavati

Now then the (mystic) correspondence (of the number of objects to the nature of Agni),--the seat, the fire-pan, the sling of the gold plate, the fire, and the gold plate,--these amount to six;--six seasons are a year, and the year is Agni: as great as Agni is, as great as is his measure, so great does this become. Two pads, that makes eight,--the Gâyatrî has eight syllables, and Agni is Gâyatra as great as Agni is, as great as is his measure, so great does this become.