Śatapatha Brāhmaṇa ("of a hundred paths") 13.3.3.9

13.3.3.9

tadāhuḥ yaddvādaśo'gniḥ syādekādaśa yūpā yathā sthūriṇā yāyāttādṛktadityekaviṃśa evāgnirbhavatyekaviṃśa stoma ekaviṃśatiryūpāstadyathā praṣṭibhiryāyāttādṛktat

As to this they say, 'If there were a twelvefold altar, and eleven stakes, it would be as if one were to drive on a cart drawn by one beast.' There are the twenty-one-fold altar, the twenty-one-fold Stoma, and twenty-one stakes: that is as when one drives with side-horses.