MĀṆḌŪKYA UPANISHAD · 8

सोऽयमात्माध्यक्षरमोङ्कारोऽधिमात्रं पादा मात्रा मात्राश्च पादा
अकार उकारो मकार इति ॥ ८॥

so’yamātmādhyakṣaramoṅkāro’dhimātraṃ pādā mātrā mātrāśca pādā
akāra ukāro makāra iti ॥ 8॥

Now this the Self, as to the imperishable Word, is OM; and as to the letters, His parts are the letters and the letters are His parts, namely, A U M.