उपमन्युतपोवर्णनं नाम चतुस्त्रिंशोऽध्यायः
Upamanyu’s penance description Chapter 34
(Upamanyu’s) mother said (to Upamanyu)
तस्मान्मन्त्रान्तरांस्त्यक्त्वा पंचाक्षरपरो भव |
तस्मिञ्जिह्वांतरगते न किंचिदिह दुर्लभम् || 46 ||
Therefore (since this mantra is powerful and can protect you always) forget other mantras and become engrossed in the five-lettered (Namah Shivaya). Once it gets on your tongue nothing is difficult to get in this world.
Chapter 32 of Shatarudra Samhita contains the same story and has some similar shlokas.