Brihadaranyaka Upanishad 4.3.20

4.3.20

tā vā asyaitā hitā nāma nāḍyo yathā keśaḥ sahasradhā bhinnastāvatāṇimnā tiṣṭhanti śuklasya nīlasya piṅgalasya haritasya lohitasya pūrṇā atha yatrainaṃ ghnantīva jinantīva hastīva vicchāyayatigartamiva patati yadeva jāgradbhayaṃ paśyati tadatrāvidyayā manyate'tha yatra deva iva rājevāhamevedam̐ sarvo'smīti manyate so'sya paramo lokaḥ

In him are those nerves called Hitâ, which are as fine as a hair split into a thousand parts, and filled with white, blue, brown, green and red (serums). (They are the seat of the subtle body, in which impressions are stored.) Now when (he feels) as if he were being killed or overpowered, or being pursued by an elephant, or falling into a pit, (in short) conjures at the time through ignorance whatever terrible things he has experienced in the waking state, (that is the dream state). And when (he becomes) a god, as it were, or a king, as it were, or thinks, 'This (universe) is myself, who am all,' that is his highest state.