BṚHADĀRAṆYAKA UPANISHAD · 1.5.8

विज्ञातं विजिज्ञास्यम् अविज्ञातम् एत एव ।
यत् किञ्च विज्ञातं वाचस् तद् रूपम् ।
वाग् घि विज्ञाता ।
वाग् एनं तद् भूत्वावति ॥

vijñātaṃ vijijñāsyam avijñātam eta eva ।
yat kiñca vijñātaṃ vācas tad rūpam ।
vāg ghi vijñātā ।
vāg enaṃ tad bhūtvāvati ॥

These are what is known, what is to be known, and what is unknown. What is known, has the form of speech, for speech is known. Speech, having become this, protects man.