The Blessed One instructs the bhikkhus on the unborn, unbecome, unmade, unconditioned.
Tatiyanibbānapaṭisaṁyutta sutta - Third Discourse on Nibbāna
Thus have I heard — Once, the Blessed One was dwelling in Sāvatthi, in Jeta's Grove, Anāthapiṇḍika's park. At that time, the Blessed One was addressing the bhikkhus on a topic related to Nibbāna, instructing, encouraging, inspiring, and gladdening them. The bhikkhus listened attentively, earnestly, and with a concentrated mind, absorbing the Dhamma with keen ears.
Then, understanding the significance of this, the Blessed One at that time expressed this inspired utterance:
"Bhikkhus, there is the unborn, unbecome, unmade, unconditioned. If there were not the unborn, unbecome, unmade, unconditioned, there would be no discernment of escape for the born, become, made, conditioned. But since there is the unborn, unbecome, unmade, unconditioned, therefore the escape from the born, become, made, conditioned is discerned."