Shatapatha Brahmana 3.6.3.3

3.6.3.3

yadveva vaisarjināni juhoti | yajño vai viṣṇuḥ sa devebhya imāṃ vikrāntiṃ vicakrame yaiṣāmiyaṃ vikrāntiridameva prathamena padena paspārāthedamantarikṣaṃ dvitīyena divamuttamenaitāmvevaiṣa etasmai viṣṇuryajño vikrāntiṃ vikramate yajjuhoti tasmādvaisarjināni juhoti

And again why he performs the Vaisargina offerings. Vishnu, forsooth, is the sacrifice; by his strides he obtained (vi-kram) for the gods that all-pervading power (vikrânti) which now belongs to them; by his first step he gained this same (earth); by the second, the region of air; and by the last, the heaven. And that same pervading power Vishnu, the sacrifice, obtains by his strides for this (sacrificer) when he sacrifices: this is why he performs the Vaisargina offerings.