Sabbath (2nd)Dutiyauposathasutta

At Sāvatthī.Sāvatthinidānaṁ.

Then a mendicant went up to the Buddha … and asked him,Atha kho aññataro bhikkhu yena bhagavā …pe… ekamantaṁ nisinno kho so bhikkhu bhagavantaṁ etadavoca: “Sir, what is the cause, what is the reason why some womb-born dragons keep the sabbath, having transformed their bodies?”“ko nu kho, bhante, hetu, ko paccayo, yena m’idhekacce jalābujā nāgā uposathaṁ upavasanti vossaṭṭhakāyā ca bhavantī”ti?

(Tell all in full.)(Sabbaṁ vitthāretabbaṁ.)