Shiva Purana
शिवो वा प्रणवो ह्येष प्रणवो वा शिवः स्मृतः ॥ वाच्यवाचकयोर्भेदो नात्यन्तं विद्यते यतः ॥ ६१ ॥
Śiva is Praṇava, Praṇava is Śiva since there is no difference between Śiva and Praṇava that expresses Śiva.
english translation
zivo vA praNavo hyeSa praNavo vA zivaH smRtaH ॥ vAcyavAcakayorbhedo nAtyantaM vidyate yataH ॥ 61 ॥
hk transliteration by Sanscript