Shatapatha Brahmana 10.3.1.4

10.3.1.4

vāganuṣṭubiti tadya eva vāco mahimā yadvīryaṃ tadetatsahasraṃ vāca evaitadvīryaṃ yaddhyasya cinvato vāgutkrāmettata evaiṣo'gnirna cīyetaitenaivāsya rūpeṇa sahasrameṣo'nuṣṭubhaḥ saṃcito bhavati

'The Anushtubh is the voice,'--thus, whatever power, whatever vigour there is in the voice that is this one thousand; and to the voice, indeed, this vigour belongs, for were the voice 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 Anushtubhs.