Śatapatha Brāhmaṇa ("of a hundred paths") 13.5.4.26

13.5.4.26

atha yadekaviṃśatirbhavanti ekaviṃśo vā eṣa ya eṣa tapati dvādaśa māsāḥ pañcartavastraya ime lokā asāvāditya ekaviṃśa etāmabhisampadam

And as to their being twenty-one of them,--the twenty-one-fold is he who shines yonder: twelve months, five seasons, these three worlds, and yonder sun as the twenty-first--this consummation (he thereby obtains).