Shiva Purana
तद्भस्मधारणात्कर्म स्वात्मन्यारोपितं भवेत् । अघोरेणात्ममंत्रेण बिल्वकाष्ठं प्रदाहयेत् ६२ ।
Hence, applying the ashes is tantamount to assimilating the sacred rite in one’s own Ātman. Bilva twigs shall be burnt repeating the Ātma mantra of Aghora
english translation
tadbhasmadhAraNAtkarma svAtmanyAropitaM bhavet । aghoreNAtmamaMtreNa bilvakASThaM pradAhayet 62 ।
hk transliteration by Sanscript