Dutiyauposathasutta

Sāvatthinidānaṁ. Atha kho aññataro bhikkhu yena bhagavā …pe… ekamantaṁ nisinno kho so bhikkhu bhagavantaṁ etadavoca: “ko nu kho, bhante, hetu, ko paccayo, yena m’idhekacce jalābujā nāgā uposathaṁ upavasanti vossaṭṭhakāyā ca bhavantī”ti? (Sabbaṁ vitthāretabbaṁ.) “Ayaṁ kho, bhikkhu, hetu, ayaṁ paccayo, yena m’idhekacce jalābujā nāgā uposathaṁ upavasanti vossaṭṭhakāyā ca bhavantī”ti. Catutthaṁ.

At Sāvatthī. Then a mendicant went up to the Buddha … and asked him, “Sir, what is the cause, what is the reason why some womb-born dragons keep the sabbath, having transformed their bodies?” (Tell all in full.)