Shatapatha Brahmana 9.1.1.31

9.1.1.31

daśaitānavatānānjuhoti | daśākṣarā virāḍvirāḍagnirdaśa diśo diśo 'gnirdaśa prāṇāḥ prāṇā agniryāvānagniryāvatyasya mātrā tāvataivaiṣāmetaddhanūṃṣyavatanoti

There are ten of these 'unstringing'-offerings he makes,--the Virâg consists of ten syllables, and Agni is Virâg (widely-shining or ruling); there are ten regions, and Agni is the regions; there are ten vital airs, and Agni is the vital airs: as great as Agni is, as great as is his measure, by so much he thus unstrings their bows.