Uposatha sutta: Sabbath

At one time the Buddha was staying near Sāvatthī in Jeta’s Grove, Anāthapiṇḍika’s monastery. Then a mendicant went up to the Buddha, sat down to one side, and said to him:

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

“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.”