Maitri (Maitrāyaṇīya) Upaniṣad 6.38

6.38

śarīraprādeśāṅguṣṭhamātramaṇorapyanvayaṃ dhyātvātaḥparamatāṃ gacchati atra hi sarve kāmāḥ samāhitā iti atrodāharanti aṅguṣṭhaprādeśaśarīramātraṃ pradīpapratāpavadvistridhā hi tadbrahmābhiṣṭūyamānaṃ maho devo bhuvanānyāviveśa |
oṃ namo brahmaṇe namaḥ ||

Having meditated upon him who is of the measure of a thumb or of a span within the body, more subtile than the subtile, then one goes to the supreme condition; for therein all desires are contained. On this point they quote:—

Of size of thumb or span within the body,
A light of twofold or of threefold brightness,
This Brahma who is being praised,
The great god—He has entered in all beings!

Om! Adoration to Brahma! yea, adoration!