Numbers 19:16, 18

Also, anyone in the open field who touches one who has been killed with a sword or who has died [of natural causes], or a human bone or a grave, shall be unclean for seven days.

A clean person shall take hyssop and dip it in the water and sprinkle it on the tent and on all the furnishings and on the people who were there, and on the one who touched the bone or the one who was killed or the one who died [naturally] or the grave.