Thus have I heard. At one time the Lord was staying near Savatthi in the Jeta Wood at Anathapindika's monastery. On that occasion the Lord was instructing... the bhikkhus with a Dhamma talk connected with Nibbana, and those bhikkhus... were intent on listening to Dhamma.

Then, on realizing its significance, the Lord uttered on that occasion this inspired utterance:

The uninclined is hard to see,
The truth is not easy to see;
Craving is penetrated by one who knows,
For one who sees there is nothing.

See also: Ud 8.1; Ud 8.3; Ud 8.4.