Chandogya Upanishad 3.12.8

3.12.8

ayaṃ vāva sa yo 'yam antaḥ puruṣa ākāśaḥ \
yo vai so 'ntaḥ puruṣa ākāśaḥ \\312.8\

7. Verily, what is called Brahma—that is the same as what the space outside of a person is. Verily, what the space outside of a person is— that is the same as what the space within a person is. Verily, what the space within a person is— that is the same as what the space here within the heart is. That is the Full, the Non-active. Full, non-active prosperity he obtains who knows this.