Numbers 19:16, 18

And whosoever in the open field toucheth one that is slain with a sword, or a dead body, or a bone of a man, or a grave, shall be unclean seven days.

and a clean person shall take hyssop, and dip it in the water, and sprinkle it upon the tent, and upon all the vessels, and upon the persons that were there, and upon him that touched the bone, or the slain, or the dead, or the grave