Śatapatha Brāhmaṇa ("of a hundred paths") 3.1.4.23

3.1.4.23

tāṃ vā anuṣṭubhā juhoti | saiṣānuṣṭupsatyekatriṃśadakṣarā bhavati daśa pāṇyā aṅgulayo daśa pādyā daśa prāṇā ātmaikatriṃśo yasminnete prāṇāḥ pratiṣṭhitā etāvānvai puruṣaḥ puruṣo yajñaḥ puruṣasammito yajñaḥ sa yāvāneva yajño yāvatyasya mātrā tāvantamevainayaitadāpnoti yadanuṣṭubhaikatriṃśadakṣarayā juhoti

He offers this one with an anushtubh verse. Being an anushtubh verse, it consists of thirty-one syllables. Now there are ten fingers, ten toes, ten vital airs, and the thirty-first is the body wherein those vital airs are contained; for this much constitutes man, and the sacrifice is a man, the sacrifice is of the same proportion as a man. Thus, whatever the extent of the sacrifice, whatever its measure, to that extent he takes possession of it by means of this (libation), when he offers it with an anushtubh verse of thirty-one syllables.