Śatapatha Brāhmaṇa ("of a hundred paths") 3.5.1.23

3.5.1.23

sa yaddhāryamāṇe 'gnau | uttaravediṃ vyāghārayati yadevaināmado devā abruvanpūrvāṃ tvāgnerāhutiḥ prāpsyatīti tadevaināmetatpūrvāmagnerāhutiḥ prāpnoti vāgghyeṣā nidānenātha yaduttaravedimupakirati yajñasyaiva sarvatvāya vāgghi yajño vāgu hyeṣā

And, accordingly, when he pours ghee on the high altar, while the fire is held (over it)--since the gods said to her on that occasion, 'The offering shall reach thee even before Agni'--then that offering does reach her even before (it reaches) Agni; for this (high altar) is in reality Vâk. And when he raises the high altar, it is for the completeness of the sacrifice, for the sacrifice is Vâk (speech) and that (high altar) is Vâk.