Śatapatha Brāhmaṇa ("of a hundred paths") 6.2.2.3

6.2.2.3

tasyaikaviṃśati sāmidhenyaḥ | dvādaśa māsāḥ pañca 'rtavastraya ime lokā asāvāditya ekaviṃśa eṣa prajāpatiḥ prajāpatiragniryāvānagniryāvatyasya mātrā tāvataivainametatsaminddhe

For this (animal sacrifice) there are twenty-one kindling-verses;--twelve months, five seasons, these three worlds, and yonder sun,--that is the twenty-onefold Pragâpati; and Pragâpati is Agni: as great as Agni is, as great as is his measure, by so much he thus kindles him.