Shatapatha Brahmana 10.5.4.9

10.5.4.9

tā vā etāḥ ekaviṃśatirbṛhatya ekaviṃśo vai svargo loko bṛhatī svargo lokastadeṣa svargaṃ lokamabhisampadyata ekaviṃśaṃ ca stomam bṛhatīṃ ca candaḥ

Now, these (amount to) twenty-one Brihatîs; and--the heavenly world being the twenty-one-fold and the Brihatî--this (altar) thus comes to be equal to the heavenly world, and to the twenty-one-fold Stoma and the Brihatî metre.