Numbers 19:16-18

16Also, a person might have killed someone with a sword out in the country. Or someone may have died because he was old or ill. Anyone who touches a dead body in the country will become unclean for seven days. Also if they touch a human bone, or a grave, that person will be unclean for seven days.

17Then for the person who is not clean, you must take some of the ashes of the young red cow. That is the cow to make people clean. You must put these ashes into a pot and you must mix them with water from a river.

18Then a person who is clean must take hyssop. He must put it into the water. Then he must use it to splash the water on the tent and on everything that is in it. He must also splash the water on all the people who have been there. And he must splash it on the person who touched the bone or the dead body or the grave.