Śatapatha Brāhmaṇa ("of a hundred paths") 3.2.1.40

3.2.1.40

atha yadbrāhmaṇa ityāha | anaddheva vā asyātā purā jānam bhavatīdaṃ hyāhū rakṣāṃsi yoṣitamanusacante taduta rakṣāṃsyeva reta ādadhatītyathātrāddhā jāyate yo brahmaṇo yo yajñājjāyate tasmādapi rājanyaṃ vā vaiśyaṃ vā brāhmaṇa ityeva brūyādbrahmaṇo hi jāyate yo yajñājjāyate tasmādāhurna savanakṛtaṃ hanyādenasvī haiva savanakṛteti

And as to his saying, 'Brâhman,' uncertain, as it were, is his origin heretofore; for the Rakshas, they say, pursue women here oil earth, and so the Rakshas implant their seed therein. But he, forsooth, is truly born, who is born of the Brahman (neut.), of the sacrifice: wherefore let him address even a Râganya, or a Vaisya, as Brâhman, since he who is born of the sacrifice is born of the Brahman (and hence a Brâhmana). Wherefore they say, 'Let no one slay a sacrificer of Soma; for by (slaying) a Soma-sacrificer he becomes guilty of a heinous sin!