Shiva Purana
अनादिमलसंश्लेषप्रागभावात्स्वभावतः ॥ अत्यन्तपरिशुद्धात्मेत्यतोऽयं शिव उच्यते ॥ ७॥
Due to the antecedent non-existence of the contact with the primordial Avidyā he is naturally pure-souled and is called Śiva.
english translation
anAdimalasaMzleSaprAgabhAvAtsvabhAvataH ॥ atyantaparizuddhAtmetyato'yaM ziva ucyate ॥ 7॥
hk transliteration by Sanscript