Srimad-Bhagavatam: Canto 12 - Chapter 1 - Verse 43
Sanskrit:
ततोऽक्षरसमाम्नायमसृजद् भगवानज: ।अन्तस्थोष्मस्वरस्पर्शह्रस्वदीर्घादिलक्षणम् ॥ ४३ ॥
ITRANS:
tato ’kṣara-samāmnāyamasṛjad bhagavān ajaḥantasthoṣma-svara-sparśa-hrasva-dīrghādi-lakṣaṇam
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.