Shiva Purana
ॐ कारं चैव यल्लिंगमेकं तच्च द्विधा गतम् ॥ प्रणवे चैव ॐकारनामासीत्स सदाशिवः ॥ २२ ॥
The phallus image of Śiva known as Oṃkāra is twofold. In the Praṇava, Sadāśiva came to be known as Oṃkāra.
english translation
OM kAraM caiva yalliMgamekaM tacca dvidhA gatam ॥ praNave caiva OMkAranAmAsItsa sadAzivaH ॥ 22 ॥
hk transliteration by Sanscript