Shiva Purana
वर्णपंचकरूपोयमेवं प्रणव उद्धृतः ॥ त्रिमात्रबिन्दुनादात्मा मुक्तिदो जपतां सदा ॥ १३ ॥
Thus the Praṇava of five syllables is explained by extricating the three Mātrās, Bindu and Nāda. It yields liberation to those who perform the Japa in this manner.
english translation
varNapaMcakarUpoyamevaM praNava uddhRtaH ॥ trimAtrabindunAdAtmA muktido japatAM sadA ॥ 13 ॥
hk transliteration by Sanscript