Śatapatha Brāhmaṇa ("of a hundred paths") 4.6.7.1

4.6.7.1

trayī vai vidyā | ṛco yajūṃṣi sāmānīyamevarco 'syāṃ hyarcati yo 'rcati sa vāgevarco vācā hyarcati yo 'rcati so 'ntarikṣameva yajūṃṣi dyauḥ sāmāni saiṣā trayī vidyā saumye 'dhvare prayujyate

Threefold, forsooth, is science; the Riks, the Yagus, and the Sâmans. The Riks are this (earth), since it is thereon that he who sings them, does sing them; the Riks are speech, since it is by speech that he who sings them, does sing them. And the Yagus, forsooth, are the air, and the Sâmans the sky. That same threefold science is used in the Soma-sacrifice.