Śatapatha Brāhmaṇa ("of a hundred paths") 5.5.3.1

5.5.3.1

abhiṣecanīyeneṣṭvā | keśānna vapate tadyatkeśānna vapate vīryaṃ vā etadapāṃ rasaḥ sambhṛto bhavati yenainametadabhiṣiñcati tasyābhiṣiktasya keśānprathamānprāpnoti sa yatkeśānvapetaitāṃ śriyam jihmāṃ vināśayedvyuduhyāttasmātkeśānna vapate

When he has performed the Consecration-ceremony (Abhishekanîya), he does not shave his hair. The reason why he does not shave his hair (is this):--that collected essence of the waters wherewith he is then sprinkled (anointed) is vigour, and it is the hair (of his head) that it reaches first when he is sprinkled; hence were he to shave his hair, he would cause that glory to fall off from him, and would sweep it away: therefore he does not shave his hair.