Maitri (Maitrāyaṇīya) Upaniṣad 7.10.p4

7.10.p4

4 manaḥ kāyāgnimāhanti sa prerayati mārutam |
mārutastūrasi caranmandraṃ janayati svaram ||

(4) The mind stirs up the body's fire.
The fire then sets in motion wind;
The wind then, moving through the chest,
Produces pleasurable sound.
(5) As stirred in heart by means of fire of friction,
Less is it than the least; in throat, is doubled;
And know that on the tongue-tip it is trebled;
Come forth, it is the alphabet!—They say thus.