Shiva Purana
वाच्यवाचकभावेन स्थितः साक्षात्स्वभावतः ॥ वाच्यश्शिवोप्रमेयत्वान्मंत्रस्तद्वाचकस्स्मृतः ॥ १० ॥
He is directly situated in the nature of the expressive and the expressive. Because Shiva is immeasurable in his speech the mantra is said to be his speaker.
english translation
vAcyavAcakabhAvena sthitaH sAkSAtsvabhAvataH ॥ vAcyazzivoprameyatvAnmaMtrastadvAcakassmRtaH ॥ 10 ॥
hk transliteration by Sanscript