Śatapatha Brāhmaṇa ("of a hundred paths") 4.1.4.5

4.1.4.5

so eva purodhā | tasmānna brāhmaṇaḥ sarvasyeva kṣatriyasya purodhāṃ kāmayeta saṃ hyetau sṛjete sukṛtaṃ ca duṣkṛtaṃ ca no eva kṣatriyaḥ sarvamiva brāhmaṇam purodadhīta saṃ hyevaitau sṛjete kukṛtaṃ ca duṣkṛtaṃ ca sa yattato varuṇaḥ karma cakre prasūtam brahmaṇā mitreṇa saṃ haivāsmai tadānṛdhe

Such, then, is the office of Purohita (placed foremost, domestic priest). Wherefore let not a Brâhman desire to become the Purohita of any one Kshatriya (he may meet with), as thereby righteousness and unrighteousness unite; nor should a Kshatriya make any Brâhman (he may meet with) his Purohita, as thereby righteousness and unrighteousness unite.--Whatever deed, sped by Mitra, the priesthood, Varuna thenceforward did, in that he succeeded.