Maitri (Maitrāyaṇīya) Upaniṣad 6.3

6.3

dve vāva brahmaṇo rūpe mūrtaṃ cāmūrtaṃ ca | atha yanmūrtaṃ tadasatyam yadamūrtaṃ tatsatyam tadbrahma tajjyotiḥ yajjyotiḥ sa ādityaḥ sa vā eṣa omityetadātmābhavat sa tredhātmānaṃ vyākuruta omiti tisro mātrā etābhiḥ sarvamidamotaṃ protaṃ caivāsmīti evaṃ hyāhaitadvā āditya omityevaṃ dhyāyata ātmānaṃ yuñjīteti ||

3. There are, assuredly, two forms of Brahma: the formed and the formless. Now, that which is the formed is unreal; that which is the formless is real, is Brahma, is light.

That light is the same as the sun.

Verily, that came to have _Om_ as its soul (_ātman_). He divided himself (_ātmānam_) threefold. _Om_ is three prosodial units (_a + u + m_). By means of these ‘the whole world is woven, warp and woof, across Him.’

For thus has it been said: ‘One should absorb himself, meditating that the sun is _Om_.’