Śatapatha Brāhmaṇa ("of a hundred paths") 13.4.4.11

13.4.4.11

atha yadekaviṃśatirbhavanti ekaviṃśatyaratnaya ekaviṃśo vā eṣa tapati dvādaśa māsāḥ pañcartavastraya ime lokā asāvāditya ekaviṃśaḥ so'śvamedha eṣa prajāpatirevametam prajāpatiṃ yajñaṃ kṛtsnaṃ saṃskṛtya tasminnekaviṃśatimagnīṣomīyānpaśūnālabhate teṣāṃ samānaṃ karmetyetatpūrvedyuḥ karma

And as to why there are twenty-one (stakes), twenty-one cubits long,--twenty-one-fold, indeed, is he that shines yonder --there are twelve months, five seasons, these three worlds, and yonder sun is the twenty-first, and he is the Asvamedha, and this Pragâpati. Having thus completely restored this Pragâpati, the sacrifice, he therein seizes twenty-one Agnîshomîya victims: for these there is one and the same performance, and this is the performance of the day before (the first Sutyâ).