Maitri (Maitrāyaṇīya) Upaniṣad 6.22

6.22

athānyatrāpyuktam dve vā va brahmaṇī abhidhyeye śabdaścāśabdaśca atha śabdenaivāśabdamāviṣkriyate atha tatra omiti śabdo'nenordhvamutkrānto'śabde nidhanameti athāhaiṣā gatiretadamṛtam atatsāyujyatvam nirvṛtatvam tathā ceti atha yathorṇanābhistantunordhvamutkrānto'vakāśaṃ labhatītyevaṃ vā va khalvāsāvabhidhyātā omityanenordhvamutkrāntaḥ svātantryaṃ labhate anyathā pare śabdavādinaḥ śravaṇāṅguṣṭhayogenāntarhṛdayākāśaśabdamākarṇayanti saptavidheyaṃ tasyopamā yathā nadyaḥ kiṅkiṇī kāṃsyacakrakabheka viḥkṛndikā vṛṣṭirnivāte vadatīti taṃ pṛthaglakṣaṇamatītya pare'śabde'vyakte brahmaṇyastaṃ gatāḥ tatra te'pṛthagdharmiṇo'pṛthagvivekyā yathā sampannā madhutvaṃ nānārasā ityevaṃ hyāha
dve brahmaṇi veditavye śabdabrahma parāṃ ca yat |
śabdabrahmaṇi niṣṇātaḥ paraṃ brahmādhigacchati ||

22. Now, it has elsewhere been said: 'Verily, there are two Brahmas to be meditated upon: sound and non-sound. Now, non-sound is revealed only by sound.' Now, in this case the sound-Brahma is *Om*. Ascending by it, one comes to an end in the non-sound. So one says: 'This, indeed, is the way. This is immortality. This is complete union (*sāyujyatva*) and also peacefulness (*nirvṛtatva*).'

Now, as a spider mounting up by means of his thread (*tantu*) obtains free space, thus, assuredly, indeed, does that meditator, mounting up by means of *Om*, obtain independence (*svātantrya*).

Others expound the sound[-Brahma] in a different way. By closing the ears with the thumbs they hear the sound of the space within the heart. Of it there is this sevenfold comparison: like rivers, a bell, a brazen vessel, a wheel, the croaking of frogs, rain, as when one speaks in a sheltered place.

Passing beyond this variously characterized [sound-Brahma],

men disappear in the supreme, the non-sound, the unmanifest Brahma. There they are unqualified, indistinguishable, like the various juices which have reached the condition of honey. For thus has it been said:—

There are two Brahmas to be known:
Sound-Brahma, and what higher is.
Those people who sound-Brahma know,
Unto the higher Brahma go.