Hebrews 9:13

13 The old rules said that the priest must use the blood of goats or bulls to make people clean. Or he could burn a young cow and mix the ashes with water. He would then splash the blood or the ashes over the people who were unclean. Then those people became clean again in their bodies, and they could worship God.

9:13Numbers 19:1-22 describes how the ashes of a young, red cow could make people clean for God.