Shiva Purana
शिवो वा प्रणवो ह्येष प्रणवो वा शिवः स्मृतः ॥ वाच्यवाचकयोर्भेदो नात्यंतं विद्यते यतः ॥ ७॥
Since there is not much difference between the sense and the sound Om, this is explained thus. Śiva is Praṇava or Praṇava is Śiva.
english translation
zivo vA praNavo hyeSa praNavo vA zivaH smRtaH ॥ vAcyavAcakayorbhedo nAtyaMtaM vidyate yataH ॥ 7॥
hk transliteration by Sanscript