Skandapurana 1.4.11.153

1.4.11.153

lauhaṃ ca rakṣasāṃ nāma bhūtabhavyabhavodbhavam..
guhyakāḥ sīsajaṃ liṃgaṃ nāma yogaṃ japaṃti ca.. 13.153 ..

and the Rākṣasas, an iron liṅga, [reciting the] name “Origin of Past, Present, and Future”;
the Guhyakas recite the name “Yoga” before a lead liṅga;