Shiva Purana
अस्य मात्राः समाख्याताश्चतस्रो वेदमूर्धनि ॥ अकारश्चाप्युकारश्च मकारो नाद इत्यपि ॥ २६ ॥
In the Upaniṣads its Mātrās are four A, U, M and nāda.
english translation
asya mAtrAH samAkhyAtAzcatasro vedamUrdhani ॥ akArazcApyukArazca makAro nAda ityapi ॥ 26 ॥
hk transliteration by Sanscript