Brihadaranyaka Upanishad 1.5.12

1.5.12

athaitasya manaso dyauḥ śarīraṃ jyotīrūpamasāvādityastadyāvadeva manastāvatī dyaustāvānasāvādityastau mithunam̐ samaitāṃ tataḥ prāṇojāyata sa indraḥ sa eṣo'sapatno dvitīyo vai sapatno nāsya sapatno bhavati ya evaṃ veda

Heaven is the body of this mind, and that sun is its luminous organ. And as far as the mind extends, so far extends heaven, and so far does that sun. The two were united, and from that the vital force emanated. It is the Supreme Lord.footnote{This is said for the purpose of meditation.} It is without a rival. A second being is indeed a rival. He who knows it as such has no rival.