ChU 3.12.9
अयं वाव स यो ऽयम् अन्तर्हृदय आकाशः । तद् एतत् पूर्णम् अप्रवर्ति । पूर्णम् अप्रवर्तिनीं श्रियं लभते य एवं वेद
ayaṃ vāva sa yo 'yam antarhṛdaya ākāśaḥ | tad etat pūrṇam apravarti | pūrṇam apravartinīṃ śriyaṃ labhate ya evaṃ veda
Devanagari rendered by HREEM from the edition's roman (IAST) text; the IAST is the edition's own.
That is the ether within the heart. That ether in the heart (as Brahman) is omnipresent and unchanging. He who knows this obtains omnipresent and unchangeable happiness.
Translated by F. Max Müller