Śatapatha Brāhmaṇa ("of a hundred paths") 3.2.1.31

3.2.1.31

atha na dīkṣitaḥ | kāṣṭhena vā nakhena vā kaṇḍūyeta garbho vā eṣa bhavati yo dīkṣate yo vai garbhasya kāṣṭhena vā nakhena vā kaṇḍūyedapāsyanmrityettato dīkṣitaḥ pāmano bhavitordīkṣitaṃ vā anu retāṃsi tato retāṃsi pāmanāni janitoḥ svā vai yonī reto na hinastyeṣā vā etasya svā yonirbhavati yatkṛṣṇaviṣāṇā tatho hainameṣā na hinasti tasmāddīkṣitaḥ kṛṣṇaviṣāṇayaiva kaṇḍūyeta nānyena kṛṣṇaviṣāṇāyāḥ

And let not the consecrated henceforth scratch himself either with a chip of wood or with his nail. For he who is consecrated becomes an embryo; and were any one to scratch an embryo either with a chip of wood or his nail, thereby expelling it, it would die. Thereafter the consecrated would be liable to be affected with the itch; and--offspring (retas) coming after the consecrated--that offspring would then also be liable to be born with the itch. Now his own womb does not injure its offspring, and that black deer's horn being indeed his own womb, that (horn) does not injure him; and therefore the consecrated should scratch himself with the black deer's horn and with nothing but the black deer's horn.