Taittiriya Upanishad 1.5.1

1.5.1

bhūrbhuvaḥ suvariti vā etāstisro vyāhṛtayaḥ /
tāsāmu ha smaitāṃ caturthīm /
māhācamasyaḥ pravedayate /
maha iti /
tat brahma /
sa ātmā /
aṅgānyanyā devatāḥ /
bhūriti vā ayaṃ lokaḥ /
bhuva ityantarikṣam /
suvarityasau lokaḥ // 1//

Bhūḥ, Bhuvaḥ, Suvaḥ—these three, indeed, are the Vyāhṛtis. Of them Māhācamasya knew a fourth one—Maha by name. It is Brahman; it is the Self. The other gods are the limbs. Bhūḥ, indeed, is this world. Bhuvaḥ is the intermediate space. Suvaḥ is the other world. Maha is the sun; through the sun, indeed, do all the worlds flourish. Bhūḥ, indeed, is the fire. Bhuvaḥ is the air. Suvaḥ is the sun. Maha is the moon; through the moon, indeed, all the luminaries flourish. Bhūḥ, indeed, is the Ṛg-Veda. Bhuvaḥ is the Sāma-Veda. Suvaḥ is the Yajur-Veda.