Śatapatha Brāhmaṇa ("of a hundred paths") 6.7.4.8

6.7.4.8

sa vai viṣṇukramānkrāntvā | atha tadānīmeva vātsapreṇopatiṣṭhate yathā prayāyātha tadānīmeva vimuñcettādṛktaddevānāṃ vai vidhāmanu manuṣyāstasmādu hedamuta mānuṣo grāmaḥ prayāyātha tadānīmevāvasyati

And, again, why the Vishnu-strides and the Vâtsapra rite are (performed). By the Vishnu-strides Pragâpati drove up to heaven. He saw that unyoking-place, the Vâtsapra, and unyoked thereat to prevent chafing; for when the yoked (beast) is not unloosed, it is chafed. In like manner the Sacrificer drives up to heaven by the Vishnu-strides; and unyokes by means of the Vâtsapra.