Mandukya Karika, verse 1.19

Text

विश्वस्यात्वविवक्षायामादिसामान्यमुत्कटम् ।
मात्रासंप्रतिपत्तौ स्यादाप्तिसामान्यमेव च ॥ १९ ॥

viśvasyātvavivakṣāyāmādisāmānyamutkaṭam |
mātrāsaṃpratipattau syādāptisāmānyameva ca || 19 ||

19. When the identity of Viśva and the sound (letter) A is intended to be described, the conspicuous ground is the circumstance of each being the first (in their respective position); another reason for this identity is also the fact of the all-pervasiveness of each.

Shankara Bhashya (commentary)

When the Śruti intends to describe Viśva as of the same nature as A (अ), then the most prominent ground is seen to be the fact of each being the first, as described in the Upaniṣad discussed above. “Mātrā sampratipath” in the text means the identity of Viśva and A. Another prominent reason for such identity is their all-pervasiveness.