SB 12.6.43

tato ’kṣara-samāmnāyam
 asṛjad bhagavān ajaḥ
antasthoṣma-svara-sparśa-
 hrasva-dīrghādi-lakṣaṇam
Synonyms: 
tataḥ — from that oṁkāra; akṣara — of the different sounds; samāmnāyam — the total collection; asṛjat — created; bhagavān — the powerful demigod; ajaḥ — unborn Brahmā; anta-stha — as the semivowels; uṣma — sibilants; svara — vowels; sparśa — and consonant stops; hrasva-dīrgha — in short and long forms; ādi — and so on; lakṣaṇam — characterized.
Translation: 
From that oṁkāra Lord Brahmā created all the sounds of the alphabet — the vowels, consonants, semivowels, sibilants and others — distinguished by such features as long and short measure.