Shiva Purana
स्वरः क्रियाशक्तिजातो ह्यकारस्तु मुनीश्वर ॥ इत्युक्ता प्रणवोत्पत्तिः पंचब्रह्मोद्भवं शृणु ॥ ५७ ॥
The vowel is born of the power of action and the syllable is the syllable, O lord of the sages. This is the origin of the oṁkāra and hear the origin of the five Brahmans.
english translation
svaraH kriyAzaktijAto hyakArastu munIzvara ॥ ityuktA praNavotpattiH paMcabrahmodbhavaM zRNu ॥ 57 ॥
hk transliteration by Sanscript