Shiva Purana
हुत्वा पूर्णाहुतिं तेषां ततः कुर्यात्प्रदीपनम् ॥ ओंकारादनु हुंकारं ततो मूलं फडंतकम् ॥ २० ॥
After offering complete oblations to the demigods one should then set fire to the sacrificial fire. Omkara followed by Humkara and then the root Phadantaka.
english translation
hutvA pUrNAhutiM teSAM tataH kuryAtpradIpanam ॥ oMkArAdanu huMkAraM tato mUlaM phaDaMtakam ॥ 20 ॥
hk transliteration by Sanscript