“Mendicant, it’s when some egg-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 egg-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 egg-born dragons keep the sabbath, having transformed their bodies.”

Linked Discourses 29.4 1. Dragons 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.)

Linked Discourses 29.5 1. Dragons Sabbath (3rd)