Shiva Purana
दर्भाग्नौ वा दहेत्काष्ठं शिवमंत्रं समुच्चरन् । सम्यक्संशोध्य वस्त्रेण नवकुंभे निधापयेत् ६५ ।
Or the twigs shall be burnt in Darbha fire repeating Śiva mantra. After straining the ashes with cloth (the fire powder) shall be put in a new pot.
english translation
darbhAgnau vA dahetkASThaM zivamaMtraM samuccaran । samyaksaMzodhya vastreNa navakuMbhe nidhApayet 65 ।
hk transliteration by Sanscript