Shiva Purana

siva.sh/siva-purana/vishweshwara-samhita/18/61

वेदाग्निजं तथा भस्म तत्कर्मांतेषु धारयेत् । मंत्रेण क्रियया जन्यं कर्माग्नौ भस्मरूपधृक् ६१ ।

sanskrit

The ashes resulting from Vedic rites in fire shall be smeared over the forehead at the end of the rites. Since the ashes are purified by the mantras the rite itself takes the form of the ashes.

english translation

vedAgnijaM tathA bhasma tatkarmAMteSu dhArayet | maMtreNa kriyayA janyaM karmAgnau bhasmarUpadhRk 61 |

hk transliteration by Sanscript