MĀṆḌŪKYA UPANISHAD · 11

सुषुप्तस्थानः प्राज्ञो मकारस्तृतीया मात्रा मितेरपीतेर्वा
मिनोति ह वा इदं सर्वमपीतिश्च भवति य एवं वेद ॥ ११॥

suṣuptasthānaḥ prājño makārastṛtīyā mātrā miterapītervā
minoti ha vā idaṃ sarvamapītiśca bhavati ya evaṃ veda ॥ 11॥

The Sleeper, Prajna, the Lord of Wisdom, He is M, the third letter, because of Measure and Finality; he that knoweth Him for such measureth with himself the Universe and becometh the departure into the Eternal.