Shiva Purana
प्रणवेनादिसंयुक्तं ब्राह्मणानां विशिष्यते । दीक्षायुक्तं गुरोर्ग्राह्यं मंत्रं ह्यथ फलाप्तये 1-11-४० ।
Repetition of the five-syllabled mantra (Namaśśivāya) by a brahmin is specially efficacious with the Om (Praṇava) prefixed. A mantra must be received from a preceptor with proper initiation for the acquisition of the desired fruit.
english translation
praNavenAdisaMyuktaM brAhmaNAnAM viziSyate । dIkSAyuktaM gurorgrAhyaM maMtraM hyatha phalAptaye 1-11-40 ।
hk transliteration by Sanscript