Vishnupurana 4.3.12

4.3.12

purukutsāya ca saṃtativicchedo na bhavato bhaviṣyatīty uragapatayo varaṃ daduḥ ||

Narmadá accordingly went to Purukutsa, and conducted him to the regions below the earth, where, being filled with the might of the deity, he destroyed the Gandharbas. He then returned to his own palace; and the snake-gods, in acknowledgment of Narmadá's services, conferred upon her as a blessing, that whosoever should think of her, and invoke her name, should never have any dread of the venom of snakes. This is the invocation; "Salutation be to Narmadá in the morning; salutation be to Narmadá at night; salutation be to thee, O Narmadá! defend me from the serpent's poison." Whoever repeats this day and night, shall never be bitten by a snake in the dark nor in entering a chamber; nor shall he who calls it to mind when he eats suffer any injury from poison, though it be mixed with his food. To Purukutsa also the snake-gods announced that the series of his descendants should never be cut off.