Śatapatha Brāhmaṇa ("of a hundred paths") 9.1.1.26

9.1.1.26

saptaitāni yajūṃṣi bhavanti | saptacitiko 'gniḥ sapta 'rtavaḥ saṃvatsaraḥ saṃvatsaro 'gniryāvānagniryāvatyasya mātrā tāvataivainametadannena prīṇāti tānyubhayānyekaviṃśatiḥ sampadyante dvādaśa māsāḥ pañca 'rtavastraya ime lokā asāvāditya ekaviṃśa etāmabhisampadam

These are seven formulas,--of seven layers the fire-altar consists, and the year consists of seven months, and Agni is the year: as great as Agni is, as great as is his measure, by so much food he thus gratifies him. These two kinds (of formulas) amount to twenty-one,--the twelve months, the five seasons, these three worlds, and yonder sun as the twenty-first (make up) this amount.