Shiva Purana
अथोर्ध्वच्छदनम्पीठपादे स्कन्दं समर्चयेत् ॥ लिंगे मूर्तिं समाकल्प्य मान्त्वया सह पूजयेत् ॥ ४२ ॥
Then one should worship Lord Skanda at the foot of the seat with the upper cover. The idol should be placed in the linga and worshiped with the mantra.
english translation
athordhvacchadanampIThapAde skandaM samarcayet ॥ liMge mUrtiM samAkalpya mAntvayA saha pUjayet ॥ 42 ॥
hk transliteration by Sanscript