Shiva Purana
रूपित्वात्सकलस्तद्वत्तस्मात्सकलनिष्कलः । निष्कलत्वान्निराकारं लिंगं तस्य समागतम् ११ ।
He is also Sakala as He has an embodied form. He is both Sakala and Niṣkala. It is in his Niṣkala aspect that the Liṅga is appropriate.
english translation
rUpitvAtsakalastadvattasmAtsakalaniSkalaH । niSkalatvAnnirAkAraM liMgaM tasya samAgatam 11 ।
hk transliteration by Sanscript