Śatapatha Brāhmaṇa ("of a hundred paths") 7.1.1.34

7.1.1.34

tā ubhayya ekaviṃśatiḥ sampadyante | dvādaśa māsāḥ pañca 'rtavastraya ime lokā asāvāditya ekaviṃśo 'muṃ tadādityamasminnagnau pratiṣṭhāpayati

Both these kinds (of bricks) amount to twenty-one;--there are twelve months, five seasons, these three worlds, and yonder sun as the twenty-first: that sun he thus establishes in this fire-altar.