Shiva Purana
पूजया चरलिंगस्य क्रमान्मुक्तो भवेद्ध्रुवम् । सर्वमुक्तं समासेन साध्यसाधनमुत्तमम् 1-18-१६० ।
By worshipping the mobile phallic image the liberation is certainly gradual. Thus I have mentioned the achievable and the excellent means of achievement.
english translation
pUjayA caraliMgasya kramAnmukto bhaveddhruvam । sarvamuktaM samAsena sAdhyasAdhanamuttamam 1-18-160 ।
hk transliteration by Sanscript