Shatapatha Brahmana 6.3.1.21

6.3.1.21

tānyetānyaṣṭau sāvitrāṇi | aṣṭākṣarā gāyatrī gāyatro 'gniryāvānagniryāvatyasya mātrā tāvataivainametadreto bhūtaṃ siñcati tāni nava bhavanti svāhākāro navamo nava diśo diśo 'gnirnava prāṇāḥ prāṇā agniryāvānagniryāvatyasya mātrā tāvataivainametadreto bhūtaṃ siñcati tāni daśa bhavantyāhutirdaśamī daśākṣarā virāḍvirāḍagnirdaśa diśo diśo 'gnirdaśa prāṇāḥ prāṇā agniryāvānagniryāvatyasya mātrā tāvattadbhavati

These then are the eight Sâvitra (formulas);--the Gâyatrî has eight syllables, and Agni is Gâyatra: as great as Agni is, as great as is his measure, by just so much he pours him out as seed. There are nine of them, the call of 'Hail' (being) the ninth,--there are nine regions, and Agni is the regions; nine vital airs, and Agni is the vital airs: as great as Agni is, as great as is. his measure, by so much he pours him out as seed. There are ten of them, the libation (being) the tenth,--the Virâg has ten syllables, and Agni is Virâg (the widely shining); there are ten regions, and Agni is the regions; ten vital airs, and Agni is the vital airs: as great as Agni is, as great as is his measure, so great does this become.