Shatapatha Brahmana 4.6.1.4

4.6.1.4

taṃ vai catuḥsraktinā pātreṇa gṛhṇāti | trayo vā ime lokāstadimāneva lokāṃstisṛbhirāpnoti prajāpatirvā atīmāṃlokāṃścaturthastatprajāpatimeva caturthyāpnoti tasmāccatuḥsraktinā pātreṇa gṛhṇāti

He draws it with a square vessel; for there are here three worlds: these three worlds he obtains by three (corners). And Pragâpati is the fourth over and above these three worlds: thus he obtains Pragâpati by the fourth (corner): therefore he draws it with a square vessel.