Shiva Purana
शिवाग्निरिति संप्रोक्तस्तेन दग्धं शिवाग्निजम् । कपिलागोमयं पूर्वं केवलं गव्यमेव वा ६३ ।
his fire is called Śivāgni. The ashes resulting therefrom are called Śivāgnija. The dung of a cow, preferably of Kapilā cow.
english translation
zivAgniriti saMproktastena dagdhaM zivAgnijam । kapilAgomayaM pUrvaM kevalaM gavyameva vA 63 ।
hk transliteration by Sanscript