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.)

“Ayaṁ kho, bhikkhu, hetu, ayaṁ paccayo, yena m’idhekacce jalābujā nāgā uposathaṁ upavasanti vossaṭṭhakāyā ca bhavantī”ti.

Catutthaṁ.