UD 8.2  Dutiya nibbānapaṭisaṁyutta sutta - Second Discourse on Nibbāna

Evaṁ me sutaṁ— ekaṁ samayaṁ bhagavā sāvatthiyaṁ viharati jetavane anāthapiṇḍikassa ārāme. Tena kho pana samayena bhagavā bhikkhū nibbānapaṭisaṁyuttāya dhammiyā kathāya sandasseti samādapeti samuttejeti sampahaṁseti. Tedha bhikkhū aṭṭhiṁ katvā manasi katvā sabbaṁ cetaso samannāharitvā ohitasotā dhammaṁ suṇanti.

Thus have I heard—At one time, the Blessed One was dwelling at Sāvatthi, in Jeta’s grove, Anāthapiṇḍika’s park. At that time, the Blessed One was instructing, encouraging, inspiring, and gladdening the bhikkhus with a discourse on the |Dhamma::teachings of the Buddha that point to the nature of reality, the ultimate truth [dhamma]| related to |Nibbāna::complete cooling, letting go of everything, deathless, freedom from calamity, the non-disintegrating [nibbāna]|. And those bhikkhus, having made it their goal, applying their minds and having collected their whole mind, were listening to the Dhamma with attentive ears.

Atha kho bhagavā etamatthaṁ viditvā tāyaṁ velāyaṁ imaṁ udānaṁ udānesi:

Then, understanding the significance of this, the Blessed One at that time expressed this inspired utterance:

“Duddasaṁ anataṁ nāma,
na hi saccaṁ sudassanaṁ;
Paṭividdhā taṇhā jānato,
passato natthi kiñcanan”ti.

“Hard to perceive is the |uninclined::unbent; epithet of Nibbāna [anata]|,
for the Truth is not easy to see;
For one who has penetrated |craving::wanting, yearning, longing, attachment, lit. thirst [taṇha]|,
who knows and sees, there is nothing [to hold to].”