Shiva Purana
प्रणवस्सर्ववेदादिः प्रणवः शिववाचकः ॥ मन्त्राधिराजराजश्च महाबीजं मनुः परः ॥ ६० ॥
Praṇava is the initial of all mantras of the Vedas. It expresses Śiva. It is the emperor of all mantras. It is the great Bīja, the greatest mantra.
english translation
praNavassarvavedAdiH praNavaH zivavAcakaH ॥ mantrAdhirAjarAjazca mahAbIjaM manuH paraH ॥ 60 ॥
hk transliteration by Sanscript