Bṛhadāraṇyaka Upaniṣad ("Great Forest Teaching") 2.3.6

2.3.6

tasya haitasya puruṣasya rūpaṃ yathā māhārajanaṃ vāso yathāpāṇḍvāvikaṃ yathendragopo yathāgnyarciryathā puṇḍarīkaṃ yathā sakṛdvidyuttam̐ sakṛdvidyutteva ha vā asya śrīrbhavati ya evaṃ vedāthāta ādeśo neti neti na hyetasmāditi netyanyatparamastyatha nāmadheyam̐ satyasya satyamiti prāṇā vai satyaṃ teṣāmeṣa satyam

And what is the appearance of that person? Like a saffron-coloured raiment, like white wool, like cochineal, like the flame of fire, like the white lotus, like sudden lightning. He who knows this, his glory is like unto sudden lightning.
Next follows the teaching (of Brahman) by No, no! for there is nothing else higher than this (if one says): 'It is not so.' Then comes the name 'the True of the True,' the senses being the True, and he (the Brahman) the True of them.

2.3.6.p1

iti dvitīyādhyāye tṛtīyaṃ brāhmaṇam