Mahavagga, Khandaka 1, Chapter 63

1. At that time there was a serpent who was aggrieved at, ashamed of, and conceived aversion for his having been born as a serpent. Now this serpent thought: 'What am I to do in order to become released from being a serpent, and quickly to obtain human nature?' Then this serpent gave himself the following answer: 'These Sakyaputtiya Samaṇas lead indeed a virtuous, tranquil, holy life; they speak the truth; they keep the precepts of morality, and are endowed with all virtues. If I could obtain pabbajjā with the Sakyaputtiya Samaṇas, I should be released from bcing a serpent and quickly obtain human nature,'

2. Then that serpent, in the shape of a youth, went to the Bhikkhus, and asked them for the pabbajjā ordination; the Bhikkhus conferred on him the pabbajjā and upasampadā ordinations.

At that time that serpent dwelt together with a certain Bhikkhu in the last Vihāra (near the boundary wall of the Jetavana). Now that Bhikkhu, having arisen in the night, at dawn, was walking up and down in the open air. When that Bhikkhu had left (the Vihāra), that serpent, who thought himself safe (from discovery), fell asleep (in his natural shape). The whole Vihāra was filled with the snake's body; his windings jutted out of the window.

3. Then that Bhikkhu thought: 'I will go back to the Vihāra,' opened the door, and saw the whole Vihāra filled with the snake's body, the windings jutting out of the window. Seeing that he was terrified and cried out. The Bhikkhus ran up, and said to that Bhikkhu: 'Why did you cry out, friend?' 'This whole Vihāra, friends, is filled with a snake's body; the windings jut out of the window.'

Then that serpent awoke from that noise and sat down on his seat. The Bhikkhus said to him: 'Who are you, friend?' 'I am a serpent, reverend Sirs.' 'And why have you done such a thing, friend?' Then that Nāga told the whole matter to the Bhikkhus; the Bhikkhus told it to the Blessed One.

4. In consequence of that and on this occasion the Blessed One, having ordered the fraternity of Bhikkhus to assemble, said to that serpent: 'You serpents are not capable of (spiritual) growth in this doctrine and discipline. However, serpent, go and observe fast on the fourteenth, fifteenth, and eighth day of each half month; thus will you be released from being a serpent and quickly obtain human nature.'

Then that serpent, who thought, 'I am not capable of (spiritual) growth in this doctrine and discipline,' became sad and sorrowful, shed tears, made an outcry, and went away.

5. Then the Blessed One said to the Bhikkhus:

'There are two occasions, O Bhikkhus, on which a serpent (who has assumed human shape) manifests his true nature: when he has sexual intercourse with a female of his species, and if he thinks himself safe (from discovery) and falls asleep. These, O Bhikkhus, are the two occasions on which a serpent manifests his true nature.

“Let an animal, O Bhikkhus, that has not received the upasampadā ordination, not receive it; if it has received it, let it be expelled (from the fraternity).”