Muṇḍaka Upaniṣad 2.2.2

2.2.2

1.2.2
yadā lelāyate hyarciḥ samiddhe havyavāhane| tadājyabhāgāvantareṇāhutīḥ pratipādayet ||

2. What is bright, what is smaller than the small, in what are centred all the world and those that live in them is this immortal Brahman. That is prana, that is speech and mind. That is true and immortal; good looking youth. Strike thy mind upon that which should be struck by the mind.

Shankara’s Commentary:
Com.—Besides it is archimat, bright, because it is by the light of the Brahman that the sun, etc., shine; again it is subtler than the subtlest (i.e.,) grain, etc. From the particle cha, it is suggested that it is bigger than the biggest, such as earth, etc. In whom all the worlds such as earth, etc., are fixed and men and the rest, inhabitants of those worlds; for all are well-known to depend upon “Intelligence”, i. e., Brahman; this immortal Brahman on which all depend is prana, speech, mind and all the instruments. It is their internal intelligence for the whole combination of prana, senses, etc., is dependent upon that intelligence, according to the Sruti ‘It is the prana of prana, etc. This immortal Brahman which is the internal intelligence of prana, etc., is true and, therefore, endless. Veddhavyam, should he seized by the mind. The meaning is that the mind should be concentrated upon the Brahman. This being so, O good looking youth, strike that, i.e., concentrate your mind upon that Brahman.