Śatapatha Brāhmaṇa ("of a hundred paths") 3.2.1.3

3.2.1.3

yadyu eka bhavati | tadeṣāṃ lokānāṃ rūpaṃ tadenameṣu lokeṣvadhi dīkṣayati yāni śuklāni tāni divo rūpaṃ yāni kṛṣṇāni tānyasyai yadi vetarathā yānyeva kṛṣṇāni tāni divo rūpaṃyāni śuklāni tānyasyai yānyeva babhrūṇīva harīṇi tānyantarikṣasya rūpaṃ tadenameṣu lokeṣvadhi dīkṣayati

But if there be only one (skin), then it is an image of these (three) worlds; then he consecrates him on these (three) worlds. Those (hairs) which are white are an image of the sky; those which are black are (an image) of this (earth);--or, if he likes, conversely: those which are black are an image of the sky, and those which are white are (an image) of this (earth). Those which are of a brownish yellow colour, are an image of the atmosphere. Thus he consecrates him on these (three) worlds.