Śatapatha Brāhmaṇa ("of a hundred paths") 11.5.8

Verily, in the beginning, Pragâpati alone was here. He desired, 'May I exist, may I be generated.' He wearied himself and performed fervid devotions: from him, thus wearied and heated, the three worlds were created--the earth, the air, and the sky.

He heated these three worlds, and from them, thus heated, three lights (gyotis) were produced--Agni (the fire), he who blows here (Vâyu), and Sûrya (the sun).

He heated these three lights, and from them, thus heated, the three Vedas were produced--the Rig-veda from Agni, the Yagur-veda from Vâyu, and the Sâma-veda from Sûrya.

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.

The gods spake unto Pragâpati, 'If our sacrifice were to fail in respect of either the Rik, or the Yagus, or the Sâman, whereby should we heal it?'

He spake, ‘If (it were to fail) in respect of the Rik, ye should take ghee by four ladlings and offer it in the Gârhapatya fire with 'Bhûh!' and if in respect of the Yagus, ye should take ghee by four ladlings and offer it in the Âgnîdhrîya--or in the Anvâhâryapakana in the case of a Haviryagña--with 'Bhuvah!' and if in respect of the Sâman, ye should take ghee by four ladlings and offer it in the Âhavanîya with 'Svar!' But if it should not be known (where the mistake has occurred), ye should make offering in the Âhavanîya after uttering rapidly all (the three sacred words): thus one heals the Rig-veda by the Rig-veda, the Yagur-veda by the Yagur-veda, and the Sâma-veda by the Sâma-veda;--even as one would put together joint with joint, so does he put together (the broken part of the sacrifice) whoever heals it by means of these (three sacred words). But if he heals it in any other way than this, it would be just as if one tried to put together something that is broken with something else that is broken, or as if one were to apply some poison as lotion to a broken part. Let him therefore appoint only one who knows this (to officiate as) his Brahman, and not one who does not know this.

As to this they say, 'Seeing that the work of the Hotri is performed with the Rig-veda, that of the Adhvaryu with the Yagur-veda, and that of the Udgâtri with the Sâma-veda, wherewith then is the work of the Brahman (performed)?' Let him reply, 'With that threefold science.'