Chandogya Upanishad 8.1.2

8.1.2

taṃ ced brūyur yad idam asmin brahmapure daharaṃ puṇḍarīkaṃ veśma daharo asminn antarākāśaḥ kiṃ tad atra vidyate yad anveṣṭavyaṃ yad vāva vijijñāsitavyam iti \
sa brūyāt \\801.2\

2. If they [i.e. the pupils] should say to him: 'This abode,

the small lotus-flower that is here in this city of Brahma, and the small space within that—what is there there which should be searched out, which assuredly one should desire to understand?’ he should say: ‘As far, verily, as this world-space (_ayam ākāśa_) extends, so far extends the space within the heart. Within it, indeed, are contained both heaven and earth, both fire and wind, both sun and moon, lightning and the stars, both what one possesses here and what one does not possess; everything here is contained within it.’