Shiva Purana
तपस्तेपेति संप्रीत्या संस्मरन्मनसा शिवम् ॥ पंचाक्षरं मनुं शंभोर्जपन्सर्वो त्तमोत्तमम् ॥ ३ ॥
Repeating the five-syllabled mantra of Śiva the most excellent of all the mantras and mentally remembering Śiva with pleasure he performed the penance.
english translation
tapastepeti saMprItyA saMsmaranmanasA zivam ॥ paMcAkSaraM manuM zaMbhorjapansarvo ttamottamam ॥ 3 ॥
hk transliteration by Sanscript