Skandapurana 1.3.7.91

1.3.7.91

pṛṣṭhakaṃṭhorujānvaṃtaḥ kamaṭhasya mahātmanaḥ..
tathāsau parvataśreṣṭho vajrasāramayo dṛḍhaḥ..
ubhayorgharṣaṇādeva vaḍavāgniḥ samutthitaḥ.. 9.91 ..

Between the back, neck, thighs, and knees of that great-souled tortoise, / that best of mountains, firm and made of the essence of diamond, [rubbed]; / and from the friction of both, the submarine fire arose.