Manava Dharmashastra 2.76

2.76

pūrvasya vidherarthavādaḥ | akṣaratrayasamāhārarūpa oṃkārastatraikaikasya utpattimāha | vedatrayāntribhyo vedebhyaḥ niraduhaduddhṛtavānyathā dadhno ghṛtamuddhriyate | na kevalamakṣaratrayaṃ yāvadidam aparaṃ bhūrbhuvaḥsvariti |

Out of the three Vedas, Prajāpati milked the letter ‘a,’ the letter ‘u’ and the letter ‘m’; as also the syllables ‘bhūḥ-bhuvaḥ-svaḥ.’—(76)

Medhātithi’s commentary (manubhāṣya):
This verse is a valedictory supplement to the foregoing injunction.

The syllable ‘oṁ’ is an aggregate of the three letters ‘a,’ ‘u’ and ‘m’; and the present verse describes the origin of each of these.

‘Out of the three Vedas’—from the three Vedas.

‘Milked’—churned out; just as butter is churned out of the curd.

Not only the three letters, but also something else, in the shape of the syllables ‘bhūḥ-bhuvaḥ-svaḥ.’—(76)

This verse is quoted in Aparārka (p. 33), as laying down the exact form of the Praṇava and of the three Mahāvyāhṛtis.

Comparative notes by various authors
Viṣṇu-Smṛti, 55.10.—[The same words as Manu.]

Baudhāyana-Dharmasūtra, 2. 10. 69.—‘The Praṇava is the soul of the Veda.’