Mandukya Karika, verse 1.26


प्रणवो ह्यपरं ब्रह्म प्रणवश्च परः स्मृतः ।
अपूर्वोऽनन्तरोऽबाह्योऽनपरः प्रणवोऽव्ययः ॥ २६ ॥

praṇavo hyaparaṃ brahma praṇavaśca paraḥ smṛtaḥ |
apūrvo'nantaro'bāhyo'naparaḥ praṇavo'vyayaḥ || 26 ||

26. (The sacred syllable) Aum verily the Lower Brahman, and it is also admitted to be the Supreme Brahman. Aum is without beginning (cause), unique, without anything outside itself, unrelated to any effect and changeless.

Shankara Bhashya (commentary)

Aum is both the Lower1 Brahman and the Supreme Turīya. When from the highest standpoint, the sounds and quarters disappear (in the soundless Aum) it is verily the same as the Supreme Brahman. It is without cause because no cause can be predicated of it. It is unique because nothing else, belonging to any other species-separate from it, exists. Similarly nothing else exists outside it. It is further not related to any effect (because it is not the cause of anything). It is without cause and exists everywhere, both inside and outside, like salt in the water of the ocean.