Mandukya Karika, verse 1.23


अकारो नयते विश्वमुकारश्चापि तैजसम् ।
मकारश्च पुनः प्राज्ञं नामात्रे विद्यते गतिः ॥ २३ ॥

akāro nayate viśvamukāraścāpi taijasam |
makāraśca punaḥ prājñaṃ nāmātre vidyate gatiḥ || 23 ||

23. The sound (letter) A helps its worshipper to attain to Viśva, U to Taijasa, and M to Prājña. In the “Soundless” there is no attainment.

Shankara Bhashya (commentary)

Having identified the quarters of Ātman with the sounds (letters) of Aum, on account of the common features stated above, he who realises the nature of the sound Aum, described above, and meditates upon it, attains to Viśva through the help of A. The meaning is that he who meditates on Aum having1 for his support A becomes Vaiśvānara.2 Similarly the meditator of U becomes Taijasa.3 Again the sound M leads its meditator to Prājña.4 But when M too disappears, causality5 itself is negated. Therefore about such Aum, which thus becomes soundless,6 no7 attainment can be predicated.