Śatapatha Brāhmaṇa ("of a hundred paths") 1.6.4.15

1.6.4.15

atha yathaiva purā | kevalīroṣadhīraśnanti kevalīrapaḥ pibanti tāḥ kevalameva payo duhra eva tadeṣa vai somo rājā devānāmannaṃ yaccandramāḥ sa yatraiṣa etāṃ rātriṃ na purastānna paścāddadṛśe tadimaṃ lokamāgacati sa ihāpaścauṣadhīśca praviśati tadenamadbhya oṣadhibhyaḥ saṃbhṛtyāhutibhyo 'dhi janayati sa eṣa āhutibhyo jātaḥ paścāddadṛśe

But as they (the cows), previously (to the new moon), eat mere plants (not imbued with the moon or Soma), and drink mere water, and yield mere milk,--so that (milk which they offer on the day before new moon, is not imbued with Soma, is ordinary milk). For king Soma, the food of the gods, indeed, is no other than the moon. When he is not seen that night either in the east or in the west, then he visits this world, and here enters into the waters and plants. Having then collected him from the water and plants, he (the performer of the Sânnâyya) causes him to be reproduced from out of the libations; and he (Soma, the moon), being reproduced from the libations, becomes visible in the western sky.