Catutthauposatha sutta: Sabbath (4th)

At Sāvatthī.

Seated to one side, that mendicant said to the Buddha:

“Sir, what is the cause, what is the reason why some spontaneously-born dragons keep the sabbath, having transformed their bodies?”

“Mendicant, it’s when some spontaneously-born dragons think: ‘In the past we did both kinds of deeds by body, speech, and mind. When the body broke up, after death, we were reborn in the company of the spontaneously-born dragons. If today we do good things by body, speech, and mind, when the body breaks up, after death, we may be reborn in a good place, a heavenly realm. Come, let us do good things by way of body, speech, and mind.’ This is the cause, this is the reason why some spontaneously-born dragons keep the sabbath, having transformed their bodies.”