It is - Katha Upanishad
naiva vācā na manasā prāptum śakyō na cakṣuṣā
astīti bruvato’nyatra katham tadupalabhyatē
Katha Upanishad, 6.12
astīti bruvato’nyatra katham tadupalabhyatē
Katha Upanishad, 6.12
Not by speech, nor by mind, nor by eye is it to be obtained.
How else can it be known, other than to tell itself, “It is!”