Śatapatha Brāhmaṇa ("of a hundred paths") 4.6.7.19

4.6.7.19

vāgevarcaśca sāmāni ca | mana eva yajūṃṣi sa ya ṛcā ca sāmnā ca caranti vākte bhavantyatha ye yajuṣā caranti manaste bhavanti tasmānnānabhipreṣitamadhvaryuṇā kiṃ cana kriyate yadaivādhvaryurāhānubrūhi yajetyathaiva te kurvanti ya ṛcā kurvanti yadaivādhvaryurāha somaḥ pavata upāvartadhvamityathaiva te kurvanti ye sāmnā kurvanti no hyanabhigatam mananā vāgvadati

The Riks and Sâmans, forsooth, are speech, and the Yagus are the mind; and so those who perform with the Rik and Sâman are speech, and those who perform with the Yagus are the mind. Hence nothing whatever is done, unless ordered by the Adhvaryu: when the Adhvaryu says, 'Recite (the invitatory prayer)! Pronounce the offering prayer!' then those who perform with the Rik perform it. And when the Adhvaryu says, 'Soma becometh pure: turn ye back!' then those who perform with the Sâman perform it,--for speech speaks not but what is conceived by the mind.