Kena Upanishad Verse 2.1
The second Khanda begins with a subtle warning against intellectual pride. If a seeker thinks they know Brahman fully, they have understood very little. Because Brahman is infinite and the subject of all experience, it cannot be categorized as a known object.
Sanskrit Verse
यदि मन्यसे सुवेदेति दहरमेवापि नूनम् |
त्वं वेत्थ ब्रह्मणो रूपम् यदस्य त्वं यदस्य देवेष्वथ नु मीमाँस्यमेव ते मन्ये विदितम् ॥ ९ ॥
Transliteration
yadi manyase suvedeti daharamevāpi nūnam |
tvaṃ vettha brahmaṇo rūpam yadasya tvaṃ yadasya deveṣvatha nu mīmām̐syameva te manye viditam ॥ 9 ॥
Translation
If thou thinkest ‘I know well’ it is certainly but little—the form of the Brahman thou hast known, as also the form in the Devas. Therefore I think that what thou thinkest known is still to be ascertained.

Muni's Play