CHĀNDOGYA UPANISHAD · 8.1.1

अथ यद् इदम् अस्मिन् ब्रह्मपुरे दहरं पुण्डरीकं वेश्म दहरो ऽस्मिन्न् अन्तराकाशः ।
तस्मिन् यद् अन्तस् तद् अन्वेष्टव्यं तद् वाव विजिज्ञासितव्यम् इति ॥

atha yad idam asmin brahmapure daharaṃ puṇḍarīkaṃ veśma daharo ’sminn antarākāśaḥ ।
tasmin yad antas tad anveṣṭavyaṃ tad vāva vijijñāsitavyam iti ॥

Harih, Om. There is this city of Brahman (the body), and in it the palace, the small lotus (of the heart), and in it that small ether. Now what exists within that small ether, that is to be sought for, that is to be understood.

F. Max Müller · The Upanishads, Sacred Books of the East 1 (1879)