Śatapatha Brāhmaṇa ("of a hundred paths") 13.3.3.10

13.3.3.10

śiro vā etadyajñasya yadekaviṃśaḥ yo vā aśvamedhe trīṇi śīrṣāṇi veda śiro ha rājñām bhavatyekaviṃśo'gnirbhavatyekaviṃśa stoma ekaviṃśatiryūpā etāni vā aśvamedhe trīṇi śīrṣāṇi tāni ya evaṃ veda śiro ha rājñām bhavati yo vā aśvamedhe tisraḥ kakudo veda kakuddha rājñām bhavatyekaviṃśo'gnirbhavatyekaviṃśa stoma ekaviṃśatiryūpā etā aśvamedhe tisraḥ kakudastā ya evaṃ veda kakuddha rājñām bhavati

That twenty-one-fold one, indeed, is the head of the sacrifice; and, verily, he who knows three heads on the Asvamedha, becomes the head of kings. There are the twenty-one-fold altar, the twenty-one-fold Stoma, and twenty-one stakes: these are the three heads on the Asvamedha; and, verily, he who thus knows them becomes the head of kings. And, indeed, he who knows the three tops on the Asvamedha, becomes the top of kings;--there are the twenty-one-fold altar, the twenty-one-fold Stoma, and twenty-one stakes: these, indeed, are the three tops on the Asvamedha; and, verily, he who thus knows them becomes the top of kings.