Mandukya Upanishad, verse 10

स्वप्नस्थानस्तैजस उकारो द्वितीया मात्रोत्कर्षादुभयत्वाद्वोत्कर्षति ह वै ज्ञानसन्ततिं समानश्च भवति नास्याब्रह्मवित्कुले भवति य एवं वेद ॥ १० ॥

svapnasthānastaijasa ukāro dvitīyā mātrotkarṣādubhayatvādvotkarṣati ha vai jñānasantatiṃ samānaśca bhavati nāsyābrahmavitkule bhavati ya evaṃ veda || 10 ||

10. Taijasa, whose sphere of activity is the dream state, is U (उ), the second letter (of Aum) on account of superiority or on account of being in between the two. He who knows this attains to a superior knowledge, is treated equally by all alike and finds no one in his line who is not a knower of Brahman.

Shankara Bhashya (commentary)

He who is Taijasa having for its sphere of activity the dream state is U (उ) the second letter of Aum. What is the point of resemblance? It is thus replied: The one common feature is superiority. The letter U:is, as it were, ‘superior’¹ to A; similarly Taijasa ² is superior to Viśva. Another common feature is: the letter U (उ) is in between the letters A (अ) and M (म). Similarly Taijasa is in between Viśva and Prājña. Therefore this condition of being in the middle is the common feature. Now is described the result of this knowledge. The knowledge (of the knower of this identity) is always on the increase, i.e., his power of knowing increases considerably. He is regarded in the same way by all, i.e., his enemies, like his friends, do not envy him. Further, in his family not one is born who is not a knower of Brahman.

Anandagiri Tika (glossary)

¹ Superior—As a matter of fact, A being the first of all sounds is superior to all letters. But U coming after A may be said to be superior to A in an indirect way.

² Taijasa—Taijasa is superior to Viśva as it is associated with ideas (in dream state) whereas Viśva is associated with gross objects (in the waking state). In dream alone one realises the world as states of mind (manaḥspandana) which knowledge brings the student nearer to truth.