Śatapatha Brāhmaṇa ("of a hundred paths") 6.7.4.11

6.7.4.11

tābhyāṃ vai viparyāsameti | yathā mahāntamadhvānaṃ vimokaṃ samaśnuvīta tādṛktatsa vai purastāccopariṣṭāccobhe viṣṇukramavātsapre samasyatyaharvai viṣṇukramā rātrirvātsapramemetadvā idaṃ sarvam prajāpatiḥ prajanayiṣyaṃśca prajanayitvā cāhorātrābhyāmubhayataḥ parigṛhṇāti

Only for one half of the year he strides the Vishnu-strides, and for one half he worships the fire with the Vâtsapra; for the world of heaven is in the midst of the year: thus were he to stride for less than half (a year), he would not reach that world of heaven; and were he to do so for more than half (a year), he would pass beyond that world of heaven and lose it; but when he strides for one half, and worships the fire for one half, he unyokes forthwith after reaching the world of heaven.