Shatapatha Brahmana 6.7.4.12

6.7.4.12

tadāhuḥ | yadaharviṣṇukramā rātrirvātsapramathobhe evāhanbhavato na rātryāṃ kathamasyāpi rātryāṃ kṛte bhavata ityetadvā ene ado dīkṣamāṇaḥ purastādaparāhṇa ubhe samasyati rātrirhaitadyadaparāhṇo 'thaine etatsaṃnivaṣsyannupariṣṭātpūrvāhṇa ubhe samasyatyaharhaitadyatpūrvāhṇa evamu hāsyobhe evāhankṛte bhavata ubhe rātryām

He proceeds with these two alternately, even as one would accomplish a long way by (repeatedly) unyoking. Both before and after (the Dîkshâ), he combines both, the Vishnu-strides and the Vâtsapra; for the Vishnu-strides are the day, and the Vâtsapra the night; and Pragâpati, both when he was about to generate and when he had generated this universe, enclosed it on both sides by day and night: in like manner the Sacrificer now, both when he is about to generate and when he has generated this universe, encloses it on both sides by day and night.