Śatapatha Brāhmaṇa ("of a hundred paths") 4.5.3.6

4.5.3.6

taṃ vai catuḥsraktinā pātreṇa gṛhṇāti | trayo vā ime lokāstadimāneva lokāṃstisṛbhiḥ sraktibhirāpnotyatyevainaṃ caturthyā sraktyā recayati tasmāccatuḥsraktinā pātreṇa gṛhṇāti

He draws it in a square cup; for there are three worlds: these same worlds he gains by three corners, and by the fourth corner he makes that (Soma) to remain over;--therefore he draws it in a square cup.