Shiva Purana
यंत्ररूपेणोपदिष्टः प्रणवश्शिववाचकः॥ समष्टिः पंचवर्णानां बिंद्वाद्यं यच्चतुष्टयम् ॥ ४५ ॥
The oṁkāra, taught in the form of a yantra, denotes Shiva. The whole is the fourth of the five characters, beginning with the dot.
english translation
yaMtrarUpeNopadiSTaH praNavazzivavAcakaH॥ samaSTiH paMcavarNAnAM biMdvAdyaM yaccatuSTayam ॥ 45 ॥
hk transliteration by Sanscript