The Bodhisatta and his younger brother, after the death of their parents, lived as ascetics in leaf huts on the Gaṅgā, the elder being higher up the stream than the younger. One day, the Nāga king Maṇikaṇṭha, while walking along the river in the guise of a man, came to the hut of the younger ascetic and became his friend. Thereafter he called daily and their friendship grew apace. Maṇikaṇṭha finally became so fond of the ascetic that he put off his disguise, and encircling the other in his folds, lay thus for a short while each day, until his affection was satisfied. However, the ascetic was afraid of his Nāga shape and grew thin and pale.
The Bodhisatta noticing this, suggested that the next time Maṇikaṇṭha came, his brother should ask for the jewel that he wore round his throat. On the morrow, when the ascetic made this request, Maṇikaṇṭha hurried away. Several times this happened, and then he came no more. The ascetic was much grieved by his absence, but was comforted by the Bodhisatta.
The Buddha related this story at the Aggāḷava-