Shiva Purana
अत एव हि पूजादौ मूर्त्यात्मपरिकल्पनम् ॥ मूर्त्यात्मनि कृतं साक्षाच्छिव एव कृतं यतः ॥ १६ ॥
That is why an idol is used in the worship of Śiva, for what is done unto the idol is what is done unto Śiva himself.
english translation
ata eva hi pUjAdau mUrtyAtmaparikalpanam ॥ mUrtyAtmani kRtaM sAkSAcchiva eva kRtaM yataH ॥ 16 ॥
hk transliteration by Sanscript