Chandogya Upanishad 3.12.9

3.12.9

ayaṃ vāva sa yo ayam antarhṛdaya ākāśaḥ \
tad etat pūrṇam apravarti \
pūrṇam apravartinīṃ śriyaṃ labhate ya evaṃ veda \\312.9\

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.