Jabali Upanishad — Mantra 7
सर्वे देवा भस्म रुद्राः सर्व देवा भस्म वासुदेवाः। तस्माद्भस्मना धारयेद्विप्रः सर्वकर्मसु॥
Sarve devā bhasma rudrāḥ sarva devā bhasma vāsudevāḥ, tasmād bhasmanā dhārayed vipraḥ sarva-karmasu
All the gods become ash in Rudra, all the gods become ash in Vasudeva — therefore the wise one should wear sacred ash in all his acts.