Śatapatha Brāhmaṇa ("of a hundred paths") 3.6.1.1

3.6.1.1

udaramevāsya sadaḥ | tasmātsadasi bhakṣayanti yaddhīdaṃ kiṃ cāśnantyudara evedaṃ sarvam pratitiṣṭhatyatha yadasminviśve devā asīdaṃstasmātsado nāma ta u evāsminnete brāhmaṇā viśvagotrāḥ sīdantyaindraṃ devatayā

The Sadas is no other than his (Vishnu, the sacrifice's) belly; therefore they feed (drink) in the Sadas; for whatever food is eaten here on earth, all that settles down in the belly. And because all the gods sat (sad) in it, therefore it is called sadas: and so do these Brâhmans of every family now sit therein. By way of deity it belongs to Indra.