Śatapatha Brāhmaṇa ("of a hundred paths") 11.5.8.4

11.5.8.4

sa imāṃstrīnvedānabhitatāpa tebhyastaptebhyastrīṇi śukrāṇyajāyanta bhūrityṛgvedādbhuva iti yajurvedātsvariti sāmavedāttadṛgvedenaiva hotramakurvata yajurvedenādhvaryavaṃ sāmavedenodgīthaṃ yadeva trayyai vidyāyai śukraṃ tena brahmatvamathoccakrāma

He heated these three Vedas, and from them, thus heated, three luminous essences were produced-- 'bhûh' from the Rig-veda, 'bhuvah' from the Yagur-veda, and 'svar' from the Sâma-veda. And with the Rig-veda they then performed the work of the Hotri priest, with the Yagur-veda the work of the Adhvaryu, and with the Sâma-veda the work of the Udgâtri; and what luminous essence there was in the threefold science, therewith the work of the Brahman priest then proceeded.