Śatapatha Brāhmaṇa ("of a hundred paths") 7.3.1.38

7.3.1.38

tadāhuḥ | kathamasyaitā ahorātrābhyāmupahitā bhavantīti dve vā ahorātre śuklaṃ ca kṛṣṇaṃ ca dva sikate śuklā ca kṛṣṇā caivamu hāsyaitā ahorātrābhyāmupahitā bhavanti

As to this they say, 'How does it come to be put on for him by means of the day and the night?' Well, day and night are two, and there are two (kinds of) seed, the white and the black: as black and white it is thus put on for him by means of the day and the night.