From AN 9.34:
Ven: Sariputta: “Reverends, extinguishment (Nibbana) is bliss!
Ven. Udayi: “But Reverend Sāriputta, what’s blissful about it, since
nothing is felt?”
Ven. Sariputta: “The fact that nothing is felt is precisely what’s
blissful about it.
Sukha or happiness for an unenlightened person is experienced when encountering pleasant feelings (from the six senses, including thoughts and the intellect) or when encountering the cessation of painful feelings (from the six senses).
But for a Buddha or arahat, sukha or bliss (in this context) is experienced when encountering neutral feelings, no feelings and Nibbana.
Please see this answer for commentaries by Nyanaponika Thera. The supporting suttas are MN 44, SN 36.5 and AN 9.34.
From Ud 8.1:
Then, on realizing the significance of that, the Blessed One on that
occasion exclaimed:
There is that dimension, monks, where there is neither earth, nor
water, nor fire, nor wind; neither dimension of the infinitude of
space, nor dimension of the infinitude of consciousness, nor dimension
of nothingness, nor dimension of neither perception nor
non-perception; neither this world, nor the next world, nor sun, nor
moon. And there, I say, there is neither coming, nor going, nor
staying; neither passing away nor arising: unestablished, unevolving,
without support [mental object]. This, just this, is the end of
stress (suffering).
From Ud 8.3:
Then, on realizing the significance of that, the Blessed One on that
occasion exclaimed:
There is, monks, an unborn — unbecome — unmade — unfabricated. If
there were not that unborn — unbecome — unmade — unfabricated, there
would not be the case that escape from the born — become — made —
fabricated would be discerned. But precisely because there is an
unborn — unbecome — unmade — unfabricated, escape from the born —
become — made — fabricated is discerned.