Shiva Purana
भस्माप्राप्तौ मुनिश्रेष्ठाः प्रवृत्ते शिवपूजने । तस्मान्मृदापि कर्तव्यं ललाटे च त्रिपुंड्रकम् ५६ ।
When the great sages (munis) attain ashes (bhasma) and engage in the worship of Lord Shiva, they must mark their forehead with the Tripundra, even with mud (if necessary).
english translation
bhasmAprAptau munizreSThAH pravRtte zivapUjane । tasmAnmRdApi kartavyaM lalATe ca tripuMDrakam 56 ।
hk transliteration by Sanscript