[127] Now at that time a certain nāga was troubled by, ashamed of, and disgusted with his birth as a nāga (nāgayoni). Then it occurred to that nāga: "By what means might I be freed from birth as a nāga and quickly regain the human state?" Then it occurred to that nāga: "These ascetics, sons of the Sakyan, are practicers of the Dhamma, of even conduct, livers of the holy life, speakers of truth, virtuous, of good character. If I were to go forth among the ascetics, sons of the Sakyan, thus I would be freed from birth as a nāga and quickly regain the human state." Then that nāga, in the form of a brahmin youth (māṇavaka), approached the bhikkhus and asked for the going forth. The bhikkhus gave him the going forth and the full ordination (upasampadā).
Now at that time that nāga was dwelling together with a certain bhikkhu in a dwelling (vihāra) on the outskirts. Then that bhikkhu, having risen in the last watch of the night, was walking up and down in the open air. Then that nāga, when that bhikkhu had gone out, relaxed and fell asleep. The whole dwelling was filled with the snake; his coils protruded out through the windows. Then that bhikkhu, pushing open the door, thinking, "I shall enter the dwelling," saw the whole dwelling filled with the snake, the coils protruding out through the windows; and having seen this, frightened, he cried out.
The bhikkhus, running up, said to that bhikkhu: "Why, friend, did you cry out?"
"Friends, this whole dwelling is filled with a snake; the coils are protruding out through the windows."
Then that nāga, awakened by that sound, sat down on his own seat. The bhikkhus said: "Who are you, friend?"
"Venerable sirs, I am a nāga."
"But why, friend, did you do such a thing?"
Then that nāga informed the bhikkhus of that matter. The bhikkhus informed the Blessed One of that matter.
Then the Blessed One, on that ground, on that occasion, having had the Saṅgha of bhikkhus convened, said to that nāga: "You nāgas are by nature incapable of growth in this Dhamma and Discipline. Go, nāga; observe the uposatha right there, on the fourteenth, the fifteenth, and the eighth day of the fortnight. Thus you will be freed from birth as a nāga and will quickly regain the human state."
Then that nāga, thinking, "So it is said that I am incapable of growth in this Dhamma and Discipline," pained and dejected, shedding tears, made an outcry and departed.
Then the Blessed One addressed the bhikkhus: "Bhikkhus, there are these two conditions for a nāga's manifestation of his own nature: when he engages in sexual intercourse with one of his own kind, and when, relaxed, he falls asleep. These, bhikkhus, are the two conditions for a nāga's manifestation of his own nature.
Bhikkhus, an animal, if not fully ordained, should not be given full ordination; if fully ordained, he should be expelled."
English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.