Śatapatha Brāhmaṇa ("of a hundred paths") 10.3.1.2

10.3.1.2

prāṇo gāyatrīti tadya eva prāṇasya mahimā yadvīryaṃ tadetatsahasram prāṇasyaivaitadvīryaṃ yaddhyasya cinvataḥ prāṇa utkkāmettata evaiṣo'gnirna cīyetaitenaivāsya rūpeṇa sahasrameṣa gāyatrīḥ saṃcito bhavati

'The Gâyatrî is the breath,'--thus, whatever power, whatever vigour there is in the breath that is this one thousand; and to the breath, indeed, this vigour belongs; for were the breath of him who builds it to pass away, this fire-altar, assuredly, would not be built: by this its form that (altar) becomes built (so as to contain) a thousand Gâyatrîs.