Śatapatha Brāhmaṇa ("of a hundred paths") 4.2.2.6

4.2.2.6

atha yasmādāgrayaṇo nāma | yāṃ vā amūṃ grāvāṇamādadāno vācaṃ yacatyatra vai sāgre 'vadattadyatsātrāgre 'vadattasmādāgrayaṇo nāma

Then as to why it is called Âgrayana. His speech which he restrains, on taking up that press-stone, spoke out again first at this (libation); and because it spoke out first (agre) at this (libation), therefore this is called the Âgrayana.