Tatiyauposathasutta

Sāvatthinidānaṁ. Ekamantaṁ nisinno kho so bhikkhu bhagavantaṁ etadavoca: “ko nu kho, bhante, hetu, ko paccayo, yena m’idhekacce saṁsedajā nāgā uposathaṁ upavasanti vossaṭṭhakāyā ca bhavantī”ti? (Sabbaṁ vitthāretabbaṁ.) “Ayaṁ kho, bhikkhu, hetu, ayaṁ paccayo, yena m’idhekacce saṁsedajā nāgā uposathaṁ upavasanti vossaṭṭhakāyā ca bhavantī”ti. Pañcamaṁ.

At Sāvatthī. Seated to one side, that mendicant said to the Buddha: “Sir, what is the cause, what is the reason why some moisture-born dragons keep the sabbath, having transformed their bodies?” (Tell all in full.)