[520] On one occasion the Blessed One was dwelling at Sāvatthī, in Jeta's Grove, in Anāthapiṇḍika's Park. Then a certain bhikkhu approached the Blessed One; having approached and paid homage to the Blessed One, he sat down to one side. Sitting to one side, that bhikkhu said this to the Blessed One: "Venerable sir, what is the cause, what is the condition, whereby some egg-born nāgas here observe the Uposatha (uposatha) and relinquish their bodies?"
"Here, bhikkhu, it occurs to some egg-born nāgas thus: 'In the past we were doers of deeds of both kinds (dvayakārino) by body, doers of deeds of both kinds by speech, doers of deeds of both kinds by mind. Having been doers of deeds of both kinds by body, doers of deeds of both kinds by speech, doers of deeds of both kinds by mind, with the breaking up of the body, after death, we were reborn in the company of the egg-born nāgas. If today we were to conduct ourselves well by body, conduct ourselves well by speech, conduct ourselves well by mind, then with the breaking up of the body, after death, we would be reborn in a good destination, a heavenly world. Come then, let us now conduct ourselves well by body, conduct ourselves well by speech, conduct ourselves well by mind.'
This, bhikkhu, is the cause, this is the condition, whereby some egg-born nāgas here observe the Uposatha and relinquish their bodies."
English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.