Shiva Purana
उपादिशं निजं मंत्रमॐकारमुरुमंगलम् । ॐकारो मन्मुखाज्जज्ञे प्रथमं मत्प्रबोधकः १६ ।
I have taught this great auspicious mantra. Oṃkāra came out of my mouth. Originally it indicated me.
english translation
upAdizaM nijaM maMtramaOMkAramurumaMgalam । OMkAro manmukhAjjajJe prathamaM matprabodhakaH 16 ।
hk transliteration by Sanscript