Śatapatha Brāhmaṇa ("of a hundred paths") 13.3.3.7

13.3.3.7

ekaviṃśo'gnirbhavati ekaviṃśa stoma ekaviṃśatiryūpā yathā vā ṛṣabhā vā vṛṣāṇo vā saṃsphurerannevamete stomāḥ samṛcante yadekaviṃśāstānyatsamarpayedārtimārcedyajamāno hanyetāsya yajñaḥ

The fire-altar is the twenty-one-fold one, the Stoma the twenty-one-fold one, and there are twenty-one sacrificial stakes; even as bulls or stallions would clash together, so do these Stomas, the twenty-one-versed, run counter to one another: were he to bring them together, the Sacrificer would suffer harm, and his sacrifice would be destroyed.