Lord Brahmā, thus contemplating, entered the water through the channel of the stem of the lotus. But in spite of entering the stem and going nearer to the navel of Viṣṇu, he could not trace out the root.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
स इत्थमुद्वीक्ष्य तदब्जनाल- नाडीभिरन्तर्जलमाविवेश । नार्वाग्गतस्तत्खरनालनाल- नाभिं विचिन्वंस्तदविन्दताजः
sa itthamudvīkṣya tadabjanāla- nāḍībhirantarjalamāviveśa | nārvāggatastatkharanālanāla- nābhiṃ vicinvaṃstadavindatājaḥ
SB 3.8.19