Śatapatha Brāhmaṇa ("of a hundred paths") 4.6.7.5

4.6.7.5

vāgevarcaśca sāmāni ca | mana eva yajūṃṣi sā yatreyaṃ vāgāsītsarvameva tatrākriyata sarvam prājñāyatātha yatra mana āsīnnaiva tatra kiṃ canākriyata na prājñāyata no hi manasā dhyāyataḥ kaścanājānāti

Both the Riks and the Sâmans are Speech, and the Yagus are the Mind. Now wherever this Speech was, there everything was done, everything was known; but wherever Mind was, there nothing whatever was done, nothing was known, for no one knows (understands) those who think in their mind.