Shiva Purana
शिवोपदिष्टमार्गेण यंत्ररूपं विभावयेत्॥ प्रणवम्परमं मन्त्राधिराजं शिवरूपिणम्॥ ४६ ॥
One should meditate on the form of the instrument by the path taught by Lord Shiva. The supreme oṁkāra, the king of mantras, in the form of Lord Śiva.
english translation
zivopadiSTamArgeNa yaMtrarUpaM vibhAvayet॥ praNavamparamaM mantrAdhirAjaM zivarUpiNam॥ 46 ॥
hk transliteration by Sanscript