Shiva Purana
उकारं चरलिंगं स्यादकारं गुरुविग्रहम् । षड्लिंगं पूजया नित्यं जीवन्मुक्तो न संशयः ११४ ।
The 'U' (pronounced as "U") is the moving Lingam, the 'A' is the form of the Guru. By worshipping the six Lingas continuously, one becomes liberated while living, without a doubt.
english translation
ukAraM caraliMgaM syAdakAraM guruvigraham । SaDliMgaM pUjayA nityaM jIvanmukto na saMzayaH 114 ।
hk transliteration by Sanscript