Shiva Purana
मनोरमे शुचौ देशे नियतः शुद्धमानसः । पंचाक्षरस्य मंत्रस्य सहस्रं द्वादशं जपेत् ४२ ।
In a beautiful and pure place, with a controlled and pure mind, one should chant the Pañcākṣarī mantra (Om Namah Shivaya) a thousand or twelve hundred times.
english translation
manorame zucau deze niyataH zuddhamAnasaH । paMcAkSarasya maMtrasya sahasraM dvAdazaM japet 42 ।
hk transliteration by Sanscript