Shatapatha Brahmana 10.5.4.6

10.5.4.6

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 one and the Brihatî (the great one)--this (altar) thus comes to be equal to the heavenly world, and to the twenty-one-fold Stoma (hymn-form) and the Brihatî metre.