Śatapatha Brāhmaṇa ("of a hundred paths") 4.2.4.18

4.2.4.18

tadvā etat | āhavanīye juhvati puroḍāśaṃ dhānāḥ karambhaṃ dadhyāmikṣāmiti tadyathā mukha āsiñcedevaṃ tadathaiṣa ekarūpa upaśeta āpa ivaiva tasmādyadanena mukhena nānārūpamaśanamaśnātyathaitena prāṇenaikarūpameva prasrāvayate 'pa ivaivātha yasmād dhruvo nāma

And, on the other hand, they offer in the Âhavanîya a sacrificial cake, parched barley-grains, porridge, sour curds, and clotted curds,--this is like pouring (food) into one's mouth. But this (libation) remains apart, (being) of one form like water. Hence while he eats the multiform food with that mouth (the fire), he lets flow from that opening the uniform (libation) like water. Then as to why it is called Dhruva.