Skanda Purana 16.186

Īśvara said: / Then one should go, O great goddess, to the unsurpassed place of the serpents, / in the western direction from Maṅkīśa, which has reached the confluence of three [rivers].

It destroys the sins of all creatures and is the great entrance to Pātāla.

Formerly, O goddess, having heard of Kṛṣṇa's death / by an arrow at Bhallatīrtha, Balabhadra then came to Prabhāsa.

Knowing the field of great efficacy that grants the fulfillment of all purposes, / and having witnessed the destruction of the Yādavas, he then attained detachment.

Emerging from his body in the form of the lord of serpents, Śeṣa, / and departing, he then reached the supreme sacred ford of the triple confluence.

Then, seeing the door of Pātāla in the form of a chasm, / having entered, he quickly went to where Ananta himself resides.

Because he entered this place in the form of a serpent, / ever since then, O ruler of the gods, it has been known as Nāgasthāna.

To the east of Nāgarāditya, where his body was cast off, / that place is renowned even today, known as Śeṣasthāna.

Therefore, having bathed, O great goddess, at that sacred ford of Trisaṅgama, / and having properly worshipped Nāgasthāna on the fifth day, fasting,

having performed a śrāddha according to one's capacity, and having given a priestly fee to a brahmin, / freed from all sorrows, he goes to the world of Rudra.

Whoever feeds a brahmin there with rice-pudding mixed with honey, / accompanied by various dishes and foodstuffs, dedicating it to Śeṣanāga, / has provided food for ten million [people]—there is no doubt about this.

Thus ends the one hundred and eighty-sixth chapter, entitled "Description of the Māhātmya of Nāgasthāna," in the first [section, named] Prabhāsakṣetramāhātmya, of the seventh [section, named] Prabhāsakhaṇḍa, in the Skandapurāṇa Mahāpurāṇa of eighty-one thousand verses.