Shatapatha Brahmana 13.5.1.6

13.5.1.6

yadvevaikaviṃśam ekaviṃśo vai puruṣo daśa hastyā aṅgulayo daśa pādyā ātmaikaviṃśastadanenaikaviṃśenātmanaitasminnekaviṃśe pratiṣṭhāyām pratitiṣṭhati tasmādekaviṃśam

And, again, as to why it is an ekavimsa day;--man is twenty-one-fold--ten fingers, ten toes, and the body (self) as the twenty-first: by means of that twenty-one-fold self he thus establishes him in the twenty-one-fold (day) as on a firm foundation,--therefore it is an ekavimsa day.