पंचाक्षरमाहात्म्यवर्णनं नाम द्वादशोऽध्यायः
Description of the greatness/importance of the five-lettered mantra – Chapter 12
Upamanyu said (to Lord Krishna)
एतावत्तु शिवज्ञानमेतावत्परमं पदम् |
यदोंनमश्शिवायेति शिववाक्यं षडक्षरम् || 20 ||
This much is the knowledge about Lord Shiva; this is the highest state, which is Om Namah Shivaya, the six-lettered Shiva-sentence.
Shloka 34 of chapter 17 in Vidyeshwara Samhita mentioned that the five-lettered mantra should always be chanted with the Pranava (Om) in the beginning.