Mudgalapurana 2.15.20

2.15.20

tasmādadhaḥ sahasrāṇi yojanāni prajāpate |
triṃśattatra svayaṃ śeṣaḥ sadā tiṣṭhati nāgapaḥ ||

Thirty thousand yojanas below that, O Prajāpati, / there Śeṣa himself, king of serpents, always remains;