About Nirvana: The Third Discourse about Nibbāna

Thus I heard: At one time the Blessed One was dwelling near Sāvatthī, in Jeta’s Wood, at Anāthapiṇḍika’s monastery. Then at that time the Blessed One was instructing, rousing, enthusing, and cheering the monks with a Dhamma talk connected with Nirvāṇa. Those monks, after making it their goal, applying their minds, considering it with all their mind, were listening to Dhamma with an attentive ear.

Then the Blessed One, having understood the significance of it, on that occasion uttered this exalted utterance: 

“There is, monks, an unborn, unbecome, unmade, unconditioned. If, monks there were not that unborn, unbecome, unmade, unconditioned, you could not know an escape here from the born, become, made, and conditioned. But because there is an unborn, unbecome, unmade, unconditioned, therefore you do know an escape from the born, become, made, and conditioned.”

~Udana 8.3