Maitri (Maitrāyaṇīya) Upaniṣad 2.2

2.2

ya eṣo bāhyāvaṣṭambhanenordhvamutkrānto vyathamāno'vyathamānastamaḥ praṇudatyeṣa ātmetyāha
bhagavānatha ya eṣa samprasādo'smāñcharīrātsamutthāya paraṃ jyotirupasampadya svena rūpeṇābhiniṣpadyata eṣa ātmeti hovācaitadamṛtamabhayametadbrahmeti ||

2. ‘Now, he who, without stopping the respiration, goes aloft and who, moving about, yet unmoving, dispels darkness—he is the Soul (Ātman). Thus said the honorable Maitri. For thus has it been said : “Now, that serene one who, rising up out of this body, reaches the highest light and appears with his own form—he is the Soul (Ātman),” said he. “That is the immortal, the fearless. That is Brahma.”