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.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
ततोऽक्षरसमाम्नायमसृजद्भगवानजः । अन्तस्थोष्मस्वरस्पर्शह्रस्वदीर्घादिलक्षणम्
tato'kṣarasamāmnāyamasṛjadbhagavānajaḥ | antasthoṣmasvarasparśahrasvadīrghādilakṣaṇam
SB 12.6.43