1At Sāvatthī. Then a certain bhikkhu approached the Blessed One, paid homage to him, sat down to one side, and said to him: "Venerable sir, what is the cause and reason why some egg-born nāgas here observe the Uposatha and relinquish concern for their bodies?" [n.285] Uposathaṁ upavasanti vossaṭṭhakāyā ca bhavanti. According to Buddhist folklore, the nāgas can undertake the precepts of virtue on the Uposatha days (see I, n. 513), and may even resolve to uphold the precepts at the cost of their lives. The classic illustration is the Campeyya Jataka (No. 506), in which the Bodhisatta, reborn as a nāga-king, maintains the Uposatha precepts even when cruelly tormented by a snake-charmer.
Spk-pṭ: "Relinquish their bodies": as they are determined to maintain the precepts, they have given up their bodies with a mind of unconcern, thinking, "Let those who have need of my skin, blood, or bones take them all."

2"Here, bhikkhus, some egg-born nāgas think thus: ‘In the past we acted ambivalently in body, speech, and mind. [n.286] Spk glosses dvayakārino with duvidhakārino, and explains that they do both wholesome and unwholesome deeds. Having done so, with the breakup of the body, after death, we were reborn in the company of egg-born nāgas. If today we practise good conduct of body, speech, and mind, then with the breakup of the body, after death, we shall be reborn in a happy destination, in a heavenly world. Come now, let us practise good conduct of body, speech, and mind.’

"This, bhikkhu, is the cause and reason why some egg-born nāgas here observe the Uposatha and relinquish concern for their bodies." sn.iii.242

1Ekaṁ samayaṁ bhagavā sāvatthiyaṁ vihārati jetavane anāthapiṇḍikassa ārāme. Atha kho aññataro bhikkhu yena bhagavā tenupasaṅkami; upasaṅkamitvā ekamantaṁ nisīdi. Ekamantaṁ nisinno kho so bhikkhu bhagavantaṁ etadavoca: "ko nu kho, bhante, hetu, ko paccayo, yena midhekacce aṇḍajā nāgā uposathaṁ upavasanti vossaṭṭhakāyā ca bhavantī"ti?

2"Idha, bhikkhu, ekaccānaṁ aṇḍajānaṁ nāgānaṁ evaṁ hoti: ‘mayaṁ kho pubbe kāyena dvayakārino ahumha, vācāya dvayakārino, manasā dvayakārino. Te mayaṁ kāyena dvayakārino, vācāya dvayakārino, manasā dvayakārino, kāyassa bhedā paraṁ maraṇā aṇḍajānaṁ nāgānaṁ sahabyataṁ upapannā. Sacajja mayaṁ kāyena sucaritaṁ careyyāma, vācāya sucaritaṁ careyyāma, manasā sucaritaṁ careyyāma, evaṁ mayaṁ kāyassa bhedā paraṁ maraṇā sugatiṁ saggaṁ lokaṁ upapajjeyyāma. Handa mayaṁ etarahi kāyena sucaritaṁ carāma, vācāya sucaritaṁ carāma, manasā sucaritaṁ carāmā’ti.

Ayaṁ kho, bhikkhu, hetu, ayaṁ paccayo, yena midhekacce aṇḍajā nāgā uposathaṁ upavasanti vossaṭṭhakāyā ca bhavantī"ti.

Tatiyaṁ.