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.
Sabbath (2nd)
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.)