The Mukhya Upaniṣads

Māṇḍūkya · Om and the Four Quarters of the Self

Mantra 11

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

In plain words

Prājña, whose field is deep sleep, is the third sound-unit M, because it measures and because all becomes absorbed in it. One who knows thus measures all this and becomes its place of absorption.