The mula vinaya text. During bhikkhu ordination (upasampada), the candidates are asked twice if they are human.
This is the reason why they ask if we are human before our ordination.
mahāvaggapāḷi p.121
tiracchānagatavatthu
nāgassa etadahosi – ‘‘ime kho samaṇā sakyaputtiyā dhammacārino samacārino brahmacārino saccavādino sīlavanto kalyāṇadhammā.
Then it occurred to that nāga: “These Sakyaputtiya recluses are righteous, impartial, chaste, truthful, virtuous, and of good character.
sace kho ahaṃ samaṇesu sakyaputtiyesu pabbajeyyaṃ, evāhaṃ nāgayoniyā ca parimucceyyaṃ, khippañca manussattaṃ paṭilabheyya’’nti.
If I were to go forth among the Sakyaputtiya recluses, I would certainly be released from the nāga realm and quickly attain human existence.”
atha kho so nāgo māṇavakavaṇṇena bhikkhū upasaṅkamitvā pabbajjaṃ yāci.
Then that nāga, in the guise of a young man, approached the bhikkhus and asked for ordination.
**taṃ bhikkhū pabbājesuṃ, upasampādesuṃ. **
**The bhikkhus gave him the going forth and ordination. **
tena kho pana samayena so nāgo aññatarena bhikkhunā saddhiṃ paccantime vihāre paṭivasati.
At that time, that nāga was staying in a border monastery with a certain bhikkhu.
atha kho so bhikkhu rattiyā paccūsasamayaṃ paccuṭṭhāya ajjhokāse caṅkamati.
Then that bhikkhu, having risen in the early morning at night, walked in the open air.
atha kho so nāgo tassa bhikkhuno nikkhante vissaṭṭho niddaṃ okkami.
Then that nāga, after that bhikkhu had left, lay down to sleep carelessly.
sabbo vihāro ahinā puṇṇo, vātapānehi bhogā nikkhantā honti.
The entire monastery was filled with snakes, and their coils extended out through the windows.
atha kho so bhikkhu vihāraṃ pavisissāmīti kavāṭaṃ paṇāmento addasa sabbaṃ vihāraṃ ahinā puṇṇaṃ, vātapānehi bhoge nikkhante, disvāna bhīto vissaramakāsi.
Then that bhikkhu, intending to enter the monastery, pushed the door open and saw the entire monastery filled with snakes, and their coils extending out through the windows, and being afraid, he cried out.
bhikkhū upadhāvitvā taṃ bhikkhuṃ etadavocuṃ – ‘‘kissa tvaṃ, āvuso, vissaramakāsī’’ti?
The bhikkhus ran up and said to that bhikkhu: “Why, sir, did you cry out?”
‘‘ayaṃ, āvuso, sabbo vihāro ahinā puṇṇo, vātapānehi bhogā nikkhantā’’ti.
“Sir, this entire monastery is filled with snakes, and their coils are extending out through the windows.”
atha kho so nāgo tena saddena paṭibujjhitvā sake āsane nisīdi.
Then that nāga, awakened by that sound, sat down on his own seat.
bhikkhū evamāhaṃsu – ‘‘kosi tvaṃ, āvuso’’ti?
The bhikkhus asked him: “Who are you, sir?”
‘‘ahaṃ, bhante, nāgo’’ti.
“Venerables, I am a nāga.”