Shiva Purana
किं तस्य बहुभिर्मंत्रैश्शास्त्रैर्वा बहुविस्तरैः ॥ यस्योन्नमः शिवायेति मन्त्रो ऽयं हृदि संस्थितः ॥ ३४ ॥
Of what avail are many mantras and Śāstras full of details to one whose heart is firmly established in the mantra “Om Namaḥ Śivāya?”
english translation
kiM tasya bahubhirmaMtraizzAstrairvA bahuvistaraiH ॥ yasyonnamaH zivAyeti mantro 'yaM hRdi saMsthitaH ॥ 34 ॥
hk transliteration by Sanscript