At the time of Buddha there was a certain Naga who was ashamed of having been born as a serpent. Now this Naga thought: “By what expedient could I become released from being born as a serpent, and quickly obtain human nature!” Then this Naga thought: “These Buddhist friars lead indeed a virtuous, tranquil, and holy life; they speak the truth; they keep the precepts of morality, and are endowed with all virtues. If, then, I could obtain admittance among the Buddhist friars, I should be released from my serpent birth, and quickly obtain human nature.”
Now this Naga, having assumed the shape of a youth, went to the Bhikshus, and asked for admittance to the Order; and the Friars granted him admittance to the Order and ordained him. At that time the Naga dwelt together with a certain Bhikshu in the last cell of the Jetavana Monastery. Now this Friar, having arisen in the night, when the dawn was at hand, was walking to and fro in the open air. When the Bhikshu had left the cell, the Naga deemed himself safe from discovery and fell asleep in his natural shape. The whole cell was filled with the snake’s body: his coils bulged out of the window.
Then the Bhikshu thought: “I will go back to the cell,” opened the door, and saw the whole cell filled with the snake. Seeing which he was terrified and cried out. The Bhikshus ran up and said to him; “Why didst thou cry out, friend?” “This who cell, friends, is filled with a snake; the coils bulge out of the window.” Then the Naga was awakened by the noise and sat down on his seat. The Bhikshus said to him: “Who art thou, friend?” “I am a Naga, reverend Sirs,” “And why hast thou done such a thing, friend?” Then the Naga told the whole matter to the Bhikshus; the Bhikshus told it to the Blessed One.
Thereupon the Blessed One, having ordered the fraternity of the Bhikshus to assemble, addressed the Naga: “Ye Nagas are not capable of spiritual growth in this doctrine and discipline. However, Naga, go thou and observe fast on the fourteenth, fifteenth, and eighth day of each half-month, thus wilt thou be released from being born as a serpent and quickly obtain human nature.” Then the Naga became sad and sorrowful, and went away.
Quoth the Blessed One: “There are two occasions, O Bhikshus, whereon a Naga, having assumed human shape, sheweth his true nature; when he hath sexual intercourse with a female of his species, and if he thinketh himself safe from discovery. Let an animal, O Bhikshus, that hath not received the ordination, not receive it; if it hath received it, let it be expelled.
Source:
Indian Serpent-Lore, J. PH. Vogel, 1926
Kotharian Lightning 