Shiva Purana
प्रणवे चैव चोंकारनामासील्लिंगमुत्तमम् ॥ परमेश्वरनामासीत्पार्थिवश्च मुनीश्वर ॥ २१॥
The excellent phallic image abides in the syllable Oṃkāra. O great sage, it also abides in the earthen phallic image in Oṃkāra Māndhātā.
english translation
praNave caiva coMkAranAmAsIlliMgamuttamam ॥ paramezvaranAmAsItpArthivazca munIzvara ॥ 21॥
hk transliteration by Sanscript