Shatapatha Brahmana 3.1.1.10

3.1.1.10

sa vai na sarveṇeva saṃvadeta | devānvā eṣa upāvartate yo dīkṣate sa devatānāmeko bhavati na vai devā sarveṇeva saṃvadante brāhmaṇena vaiva rājanyena vā vaiśyena vā te hi yajñiyāstasmādyadyenaṃ śūdreṇa saṃvādo vindedeteṣāmevaikam brūyādimamiti vicakṣvemamiti vicakṣvetyeṣa u tatra dīkṣitasyopacāraḥ

Let him not commune with every one; for he who is consecrated draws nigh to the gods, and becomes one of the deities. Now the gods do not commune with every one, but only with a Brahman, or a. Râganya, or a Vaisya; for these are able to sacrifice. Should there be occasion for him to converse with a Sûdra, let him say to one of those, 'Tell this one so and so! tell this one so and so!'
This is the rule of conduct for the consecrated in such a case.