Muṇḍaka Upaniṣad 2.2.2

2.2.2

yad arcimad yad aṇubhyo 'ṇu ca yasmiṃl lokā nihitā lokinaś ca.
tad etad akṣaraṃ brahma sa prāṇas tad u vāṅ manas.
tad etat satyaṃ tad amṛtaṃ tad veddhavyaṃ somya viddhi.

2. That which is flaming, which is subtler than the subtle,
On which the worlds are set, and their inhabitants—
That is the imperishable Brahma.
It is life (prāṇa), and It is speech and mind.
That is the real. It is immortal.
It is [a mark] to be penetrated. Penetrate It, my friend!