BṚHADĀRAṆYAKA UPANISHAD · 1.5.10

यत् किञ्चाविज्ञातं प्राणस्य तद् रूपम् ।
प्राणो ह्य् अविज्ञातः ।
प्राण एनं तद् भूत्वावति ॥

yat kiñcāvijñātaṃ prāṇasya tad rūpam ।
prāṇo hy avijñātaḥ ।
prāṇa enaṃ tad bhūtvāvati ॥

What is unknown, has the form of breath, for breath is unknown. Breath, having become this, protects man.

F. Max Müller · The Upanishads, Sacred Books of the East 15 (1884)