Shiva Purana
अंगुष्ठादावपि तथा पूजामिच्छंति केचन । लिंगकर्मणि सर्वत्र निषेधोस्ति न कर्हिचित् ३४ ।
Some authorities have recommended the worship of the phallic emblem on the thumb etc. In these rites of phallic worship, there is no sort of prohibition whatsoever.
english translation
aMguSThAdAvapi tathA pUjAmicchaMti kecana । liMgakarmaNi sarvatra niSedhosti na karhicit 34 ।
hk transliteration by Sanscript