Shiva Purana
व्यष्टिरूपेण संसिद्धं प्रणवे शिववाचके ॥ यंत्ररूपं शृणु प्राज्ञ शिवलिंगं तदेव हि ॥ १५ ॥
It is perfected in the form of a stick in the oṁkāra denoting Lord Śiva. O wise one hear the form of the instrument that is the Shiva Linga.
english translation
vyaSTirUpeNa saMsiddhaM praNave zivavAcake ॥ yaMtrarUpaM zRNu prAjJa zivaliMgaM tadeva hi ॥ 15 ॥
hk transliteration by Sanscript