Śatapatha Brāhmaṇa ("of a hundred paths") 6.2.2.4

6.2.2.4

yadvevaikaviṃśatiḥ | ekaviṃśo vai puruṣo daśa hastyā aṅgulayo daśa pādyā ātmaikaviṃśaḥ puruṣaḥ prajāpatiḥ prajāpatiragniryāvānagniryāvatyasya mātrā tāvataivainametatsaminddhe

And, again, why there are twenty-one;--man (purusha) doubtless is twenty-onefold, ten fingers of the hand, ten toes, and the body (make) the twenty-onefold man Pragâpati; and Pragâpati is Agni: as great as Agni is, as great as is his measure, by so much he thus kindles him.