Śatapatha Brāhmaṇa ("of a hundred paths") 3.9.4.15

3.9.4.15

sa indro 'bravīt | sarvaṃ vai ma idaṃ tasthānaṃ yadidaṃ kiṃ ca tiṣṭhadhvameva ma iti tā hocuḥ kiṃ nastataḥ syāditi prathamabhakṣa eva vaḥ somasya rājña iti tatheti tā asmā atiṣṭhanta tāstasthānā urasi nyagṛhṇīta tadyadenā urasi nyagṛhṇīta tasmānnigrābhyā nāma tathaivaitā etadyajamāna urasi nigṛhṇīte sa āsāmeṣa prathamabhakṣaḥ somasya rājño yannigrābhyābhirupasṛjati

Indra spake, 'Verily, all this (universe), whatsoever there is, has submitted unto me: submit ye also to me!'--They said, 'What shall be our (reward) then?'--'The first draught of king Soma shall be yours!'--'So be it!' thus they submitted to him; and they having submitted, he drew (ni-grabh) them to his breast; and because he thus drew them to his breast, therefore they are called Nigrâbhyâs. And in like manner does this sacrificer now draw them to his breast: and this is their first draught of king Soma, in that he pours Nigrâbhyâ water thereon.