Numbers 31:20

And purify all your raiment, and all that is made of skins, and all work of goats’ hair, and all things made of wood.

Cross-references

Numbers 19:14-16 details uncleanness from touching a corpse — the very situation soldiers faced; this verse commands the required purification.

Numbers 19:15 says open vessels are unclean — relevant because wood items (vessels) needed purification after corpse contact.

Numbers 19:22 states that anything touched by an unclean person becomes unclean — explaining why all items needed purification after contact with death.