Śatapatha Brāhmaṇa ("of a hundred paths") 7.2.4.25

7.2.4.25

pañcadaśodacamasānninayati pañcadaśabhirṛgbhirvapati tattriṃśattriṃśadakṣarā virāḍvirāḍu kṛtsnamannaṃ sarvamevāsminnetatkṛtsnamannaṃ dadhāti

Fifteen jarfuls of water he pours out, and with fifteen verses he sows,--that makes thirty,--the Virâg (metre) consists of thirty syllables, and the Virâg (the far-shining, or far-ruling) is the whole food: the whole food he thus puts into him.