Bamidbar (Numbers): Chukat > Ch 19
Ritual cleansing with ashes of a perfectly red unblemished cow – must be done for those who touched a human corpse, before they can come to the Mishkan
How to prepare the ashes:
The Lord spoke to Moses and Aaron, saying: This is the statute of the Torah which the Lord commanded, saying: Have the children of Israel take for you a perfectly red unblemished cow, upon which no yoke was laid. Give it to Eleazar the kohen. He will take it outside the camp and have it slaughtered in his presence. He will take from its blood with his finger and sprinkle it toward the front of the Tent of Meeting seven times. The cow will then be burned in his presence; its hide, its flesh, its blood, with its dung. The kohen will take a piece of cedar wood, hyssop, and crimson wool, and cast them into the burning of the cow. The kohen will wash his garments and bathe his flesh in water, and then he may enter the camp, and he will be unclean until evening.
The one who burns it will wash his clothes in water and cleanse his body in water, and he will be unclean until evening.
A ritually clean person will gather the cow's ashes and place them outside the camp in a clean place, and It will be as a keepsake for the congregation of the children of Israel for sprinkling water, [used] for cleansing. The one who gathers the cow's ashes will wash his clothes, and he will be unclean until evening. It will be an everlasting statute for the children of Israel and for the proselyte who resides in their midst.
How to clean oneself using the ashes:
Anyone touching the corpse of a human soul shall become unclean for seven days. On the third and seventh days, he shall cleanse himself with it, so that he can become clean. But if he does not sprinkle himself with it on the third and seventh days, he shall not become clean. Whoever touches the corpse of a human soul which dies, and he does not cleanse himself, he has defiled the Mishkan of the Lord, and that soul shall be cut off from Israel. For the sprinkling water was not sprinkled on him, so he remains unclean, and his uncleanness remains upon him.
This is the law: if a man dies in a tent, anyone entering the tent and anything in the tent shall be unclean for seven days. Any open vessel which has no seal fastened around it becomes unclean. Anyone who touches one slain by the sword, or a corpse, or a human bone or a grave, in an open field, he shall be unclean for seven days.
They shall take for that unclean person from the ashes of the burnt purification offering, and it shall be placed in a vessel [filled] with spring water. A ritually clean person shall take the hyssop and dip it into the water and sprinkle it on the tent, on all the vessels, and on the people who were in it, and on anyone who touched the bone, the slain person, the corpse, or the grave. The ritually clean person shall sprinkle on the unclean person on the third day and on the seventh day, and he shall cleanse him on the seventh day, and he shall wash his clothes and bathe in water, and he shall become ritually clean in the evening.
If a person becomes unclean and does not cleanse himself, that soul shall be cut off from the congregation, for he has defiled the Sanctuary of the Lord; the sprinkling waters were not sprinkled upon him. He is unclean. This shall be for them as a perpetual statute, and the one who sprinkles the sprinkling waters shall wash his clothes, and one who touches the sprinkling waters shall be unclean until evening. Whatever the unclean one touches shall become unclean, and anyone touching him shall be unclean until evening.