Śatapatha Brāhmaṇa ("of a hundred paths") 3.9.3.3

3.9.3.3

antareṇeṣa upāvaharati | yajño vā anastannveva yajñānna bahirdhā karoti grāvasu sammukheṣvadhinidadhāti kṣatraṃ vai somo viśo grāvāṇaḥ kṣatramevaitadviśyadhyūhati tadyatsammukhā bhavanti viśamevaitatsammukhāṃ kṣatriyamabhyavivādinīṃ karoti tasmātsammukhā bhavanti

He takes him down between the shafts; for the cart is (a means of) the sacrifice, and thus alone he does not put him outside the sacrifice. He puts him on the pressing stones lying there with their heads (mukha, mouths) towards each other; for Soma is the nobility, and the stones are the clans (people); he thereby raises the nobility over the clan. And as to why they are lying with their heads together,--he thereby makes the clan of one head (or mouth) with, and uncontentious towards, the nobles; therefore they are lying with their heads towards each other.