Śatapatha Brāhmaṇa ("of a hundred paths") 6.7.4.10

6.7.4.10

sa vā ardhameva saṃvatsarasya viṣṇukramānkramate | ardhaṃ vātsapreṇopatiṣṭhate madhye ha saṃvatsarasya svargo lokaḥ sa yatkanīyo 'rdhātkrameta na haitaṃ svargaṃ lokamabhiprāpnuyādatha yadbhūyo 'rdhātparāṅ haitaṃ svargaṃ lokamatipraṇaśyedatha yadardhaṃ kramate 'rdhamupatiṣṭhate tatsamprati svargaṃ lokamāptvā vimuñcate

Now, the Vishnu-strides indeed are the day and the night, and the Vâtsapra is the day and the night: he thus journeys for a day and a night, and takes rest for a day and a night: and hence even now when a troop of men have journeyed for a day and a night they take rest for a day and a night.