Shatapatha Brahmana 11.1.6.11

11.1.6.11

sa yadasmai devāntsasṛjānāya divevāsa tadaharakurutātha yadasmā asurāntsasṛjānāya tama ivāsa tāṃ rātrimakuruta te ahorātre

Now what daylight, as it were, there was for him, on creating the gods, of that he made the day; and what darkness, as it were, there was for him, on creating the Asuras, of that he made the night: they are these two, day and night.