Uposathasutta

sn29.3:1.1 At one time the Buddha was staying near Sāvatthī in Jeta’s Grove, Anāthapiṇḍika’s monastery.

sn29.3:1.2 Then a mendicant went up to the Buddha, sat down to one side, and said to him:

sn29.3:1.3 “Sir, what is the cause, what is the reason why some egg-born dragons keep the sabbath, having transformed their bodies?”