Śatapatha Brāhmaṇa ("of a hundred paths") 4.2.1.26

4.2.1.26

tau vai vaṣaṭkṛtau santau mantreṇa hūyate | eteno haitau tadudaśnuvāte yadenau sarvaṃ savanamanuhūyate yadvevaitau sarvaṃ savanamanuhūyata etau vai prajāpateḥ pratyakṣatamāṃ cakṣuṣī hyetau satyaṃ vai cakṣuḥ satyaṃ hi prajāpatistasmādenau sarvaṃ savanamanuhūyate

Being consecrated by Vashat, these two (libations) are offered with a prayer. Now it is because the entire Savana is offered after these two (libations) that they attain to this (distinction); and the reason why the entire Savana is offered after them, is that they are most distinctly Pragâpati's own: for they are the eyes, and the eye is the truth, and Pragâpati is the truth;--this is why the entire Savana is offered after them.