Śatapatha Brāhmaṇa ("of a hundred paths") 3.2.1.24

3.2.1.24

tatraitāmapi vācamūduḥ | upajijñāsyāṃ sa mlecastasmānna brāhmaṇo mlecedasuryā haiṣā vā natevaiṣa dviṣatāṃ sapatnānāmādatte vācaṃ te 'syāttavacasaḥ parābhavanti ya evametadveda

Such was the unintelligible speech which they then uttered,--and he (who speaks thus) is a Mlekkha (barbarian). Hence let no Brahman speak barbarous language, since such is the speech of the Asuras. Thus alone he deprives his spiteful enemies of speech; and whosoever knows this, his enemies, being deprived of speech, are undone.