Shatapatha Brahmana 5.5.3.6

5.5.3.6

sa vai nyeva vartayate keśānna vapate vīryaṃ vā etadapāṃ rasaḥ sambhṛto bhavati yenainametadabhiṣiñcati tasyābhiṣiktasya keśānprathamānprāpnoti sa yatkeśānvapetaitāṃ śriyaṃ jihmāṃ vināśayedvyuhyādatha yannivartayate tadātmanyevaitāṃ śriyaṃ niyunakti tasmānnyeva vartayate keśānna vapate tasyaiṣaiva vratacaryā bhavati yāvajjīvaṃ nāsyām pratitiṣṭhati

He only cuts down his hair, but does not shave it; for that collected essence of the waters with which he is sprinkled is vigour, and it is the hair that it reaches first when he is sprinkled. Thus were he to shave off his hair he would cause that glory to fall off from him, and would sweep it away. But when he cuts it down, he attaches that glory to his own self: therefore he only cuts down his hair, but does not shave it. This is for him a religious observance: as long as he lives he does not stand on this (earth with bare feet).