CHĀNDOGYA UPANISHAD · 3.12.7

यद् वै तद् ब्रह्मेतीदं वाव तद् यो ऽयं बहिर्धा पुरुषाद् आकाशः ।
यो वै स बहिर्धा पुरुषाद् आकाशः ॥

yad vai tad brahmetīdaṃ vāva tad yo ’yaṃ bahirdhā puruṣād ākāśaḥ ।
yo vai sa bahirdhā puruṣād ākāśaḥ ॥

The Brahman which has been thus described (as immortal with three feet in heaven, and as Gâyatrî) is the same as the ether which is around us;