After creating the total material energy, the mahat-tattva, and thereby manifesting the gigantic universal form with senses and sense organs, the Supreme Lord entered within it.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
सृष्ट्वाग्रे महदादीनि सविकाराण्यनुक्रमात् । तेभ्यो विराजमुद्धृत्य तमनु प्राविशद्विभुः
sṛṣṭvāgre mahadādīni savikārāṇyanukramāt | tebhyo virājamuddhṛtya tamanu prāviśadvibhuḥ
SB 3.7.21