Aitareya Āraṇyaka 3.1.1

3.1.1

athātaḥ saṃhitāyā upaniṣat \
pṛthivī pūrvarūpaṃ.
dyaur utta/rarūpaṃ.
vāyuḥ saṃhiteti māṇḍūkeya.
ākāśaḥ saṃhitety asya mākṣavyo vedayāñ cakre \
sa hāviparihṛto mene.
na me 'sya putreṇa samagā/d iti \
samāne vai.
tat parihṛto mena ity āgastyaḥ.
samānaṃ hy etad bhavati vāyuś cākāśaś ca \
ity adhidaivatam \
athādhyātmam \
vāk pūrvarūpaṃ.
mana uttararūpaṃ.
prāṇaḥ saṃhiteti śūravīro māṇḍūkeyaḥ \
atha hāsya putra āha jyeṣṭho.
manaḥ pūrvarūpaṃ.
vāg uttararūpaṃ.
manasā vā agre saṃkalpayaty.
atha vācā vyāharati.
tasmān mana eva pūrvarūpaṃ.
vāg uttararūpaṃ.
prāṇas tv eva saṃhiteti \
samānam enayor atra pituś ca putrasya ca \
sa eṣo 'śvarathaḥ praṣṭivāhano manovākprāṇasaṃhataḥ \
sa ya evam etāṃ saṃhitāṃ veda, saṃdhīyate prajayā paśubhir yaśasā brahmavarcasena svargeṇa lokena.
sarvam āyur eti \
iti nu māṇḍūke/yānām \\ 1 \\

The former half is the earth, the latter half the heaven, their union the air, thus says Mândukeya; their union is the ether, thus did Mâkshavya teach it.

That air is not considered independent, therefore I do not agree with his (Mandûka's) son.

Verily, the two are the same, therefore air is considered independent, thus says Âgastya. For it is the same, whether they say air or ether.

So far with reference to deities (mythologically); now with reference to the body (physiologically):

The former half is speech, the latter half is mind, their union breath (prâna), thus says Sûravîra Mândukeya.

But his eldest son said: The former half is mind, the latter half speech. For we first conceive with the mind indeed, and then we utter with speech. Therefore the former half is indeed mind, the latter half speech, but their union is really breath.

Verily, it is the same with both, the father (Mândukeya) and the son.

This (meditation as here described), joined with mind, speech, and breath, is (like) a chariot drawn by two horses and one horse between them (prashtivâhana).

And he who thus knows this union, becomes united with offspring, cattle, fame, glory of countenance, and the world of Svarga. He lives his full age.

Now all this comes from the Mândukeyas.